Where should I leave my passport and electronics if there is no safe in my apartment?

I would leave the stuff in the apartment as it is too unpractical to carry it around at all times. As you've said, backpacks are much easier to steal and it's much less risky for the thief than breaking in and searching through an empty apartment.

However, I would conceal my belongings either in a suitcase with locks or maybe hidden away between your clothes.

Passports I carry around at all times, since they are not too much in the way and they fit very nicely in travel belts, which you can carry under your clothes.

EDIT: Since there's a somewhat heated debate going on in the comments of this answer whether "hiding" your valuables actually does any good or not and this is the highest scoring answer right now, I think I should point to the answers of alephzero and unknownprotocol who explain in great detail as to why "hiding" does make sense: Leaving your stuff out in the open creates opportunities for thieves/burglars! No burglar would break into an apartment where they can see nothing from the outside if the next door neighbour has left his golden iPhone laying around on the kitchen table right next to the window, if you know what I mean.

Thieves aren't entirely stupid. They "steal from banks because that's where the money is".

If there is nothing to advertise the fact that your rented apartment happens to have a well-off but potentially careless occupant right now, they won't take the risk of breaking when they don't know what and/or who they might find inside the apartment, but go some place else where they already know there is something worth stealing and nobody (and no dogs!) at home.

Just don't create an opportunity right in front of their nose. For example, I live in a very quiet street in a small village in the UK. AFAIK there has never been a recorded crime in the 30+ years I have lived there, except for one.

A neighbour had a daughter who was at university and came home to visit one weekend. Heading back late on Sunday evening, she put one suitcase in the car, left the car door open, and went back into the house to get her other case and say goodbye.

During that five-minute period, some chancer who walked by saw the case, took it, took it out of sight of the car and below a street light, opened the case (which wasn't locked,) found nothing obviously worth stealing inside, The case was then thrown over the hedge into someone's garden, and the would be thief ran off.

The local police had a pretty good idea "who did it", based on past form (he would have been heading home from the local pub) but of course there was no proof - and indeed no actual theft took place.

That's the sort of situation you need to avoid!

According to statistics, thefts in Spain occur 6-7 times more often than burglary (541,561 vs. 83,112 in 2015), and I'd expect this ratio being even higher in popular tourist destinations like Barcelona. So yes, your belongings will be safer in the apartment than in your backpack.

I leave my stuff (passport, laptop...) in my closed suitcase.

I am not overly paranoid about it.

If there are problem with breakins or stolen goods at an AirBnB type apartment, then it would have been reported and the apartment would be removed or at least its rating greatly downgraded.

I'd recommend taking your passport with you because it's what is supposed to be your ID, if you don't have an EU one. Driver licence is not meant to be an ID, it's only a document proving that you are allowed to drive. It differs from country to country, I can't tell for sure how it looks like in Spain, but better be safe than sorry.

Unless you live in place which is know to have terrible break-in statistics, if you're the only one who have keys, your belongings are usually much safer there than taken with you.

As for passport security, it is a good idea to keep them separate from money. I personally keep wallet in my front jeans pocket, which makes it impossible for thief to take without my notice (and very hard with it, without using a lot of physical violence). But thieves in general want no attention so they prefer not using violence.

Reading this, I'm picturing the little in-room safes common at hotels. If that's what you had in mind, this is a blessing in disguise for security, because those are usually really trivial to bypass (source: I worked in a hotel and sometimes had to get into them because guests locked themselves out & needed their passports to fly home the next day)

If guests really needed to keep something safe though, we could watch it for them at the front desk where we had a real safe. You could ask around at hotels in the area if they could do that service for a small fee or give you other recommendations

If you leave expensive items in your apartment and it gets stolen, then the insurance very likely will not pay out. The mere fact that your stuff was stolen is then pretty much proof that your apartment was not secured properly against burglary. Now, whether that's actually the case or not will depend on the police report, but you're unlikely to get a favorable police report. Likely, the police report will say that the burglars got in with little effort.

That's why I always carry expensive stuff with me. I do that in a secure way such that it can only get stolen due to an armed robbery, and not by a pickpocket or some thief who could quickly snatch a bag away from me. If you're the victim of an armed robbery, then the police report will be very clear about that and the insurance will compensate you for the lost property.

Stuff like passports, wallets, mobile phones etc. can be carried in the inside pockets of your jacket. Make sure you use a jacked that has pockets with zippers. Larger stuff is best carried in a backpack that is strapped on and closed with zippers. There is still a threat from pickpockets if you're in a busy place standing still for a long time, say an overcrowded train. In such a situation it's possible for a pickpocket to get your stuff using a knife to cut through your jacket or backpack. This not something that's likely to happen in Europe but it does happen in India where people tend to get packed together like sardines in a tin can in trains and buses.

If you have simple tools [flathead, philipshead screwdriver] you can dismantle something in your apartment and put it inside. Anything big enough that will hold your money and passports. Also, hedge your bets and separate them out. I can't speak for hiding something as large as an ipad, but it's less valuable than raw cash or passports, so worry less about that.

Examples:

1. Duct tape them in a bag to the inside of a vent where the grate requires removal

2. Pull apart a lamp, put it inside

3. Peel up the carpet in a corner of the room, put it under there, put a piece of furniture over it

4. Pull out the ceiling fan in the bathroom, duct tape your stuff above it in the vent

5. Get a waterproof bag, put it in the toilet reservoir

6. Money fits nicely inside books

7. Extremely disgusting smelly socks are a wonderful place to store things that you don't want others to touch

8. Inside of food

9. Are your bedposts hollow? Put it inside there

10. Electric light switch on a wall, it's technically a precut hole in the wall, you can pull of the panel, and potentially depending on your own mechanicaly experience, move the electric box aside, tie a string to your valueables and lower it into the wall, leaving the bottom of the string tied to the electric box of the light switch, then put the cap back on

11. Plumbing under the sink, get a wrench, pull out the U shaped section, stuff it in there in a waterproof bag, for further security, turn off the water feeding the sink

I hope these help!

Yes, I will say also. We should not carry our important original documents. One of my friends was travelling with her all original documents. He had recently passed their graduation. When he was standing on the gate of train. One kids came near the gate and snatched her bag and ran away.

She could not do nothing. If there is no safe place, you can hire luggage counter and lock it when go anywhere.

I think that this problem bothers many people. Since the loss of money or passport in another country is terrible. Then you have to call the police to prove that you are yourself. I am sure that it is better not to leave money and valuables in the apartment that you are renting. I often use a money belt, like this one. In this case, if your backpack is stolen, you will have money for the first time.

Yes, your backpack can be stolen in the city, but there is a high probability that it will be stolen in the apartment that you are renting. So it was with my friend, he was on vacation in Italy and rented an apartment, and he was robbed by valuable things from the apartment. The thief even took the chocolate from the refrigerator. Agree, there is no certainty that one of the former tenants did not make a copy of the key from the front door, right?