What happens if you swab positive for a drug sample at Canadian airports even if you don’t have anything on you?

Just because your bag is triggered doesn't mean you will be in trouble. I bought a backpack at Heathrow duty free while in transit to the US which triggered a positive hit on the explosive wand - while it was still in the plastic.

It got swiped twice, detected twice, and then the inspector looked at it and just let me through, no muss no fuss.

In your case, I would get a copy of the police report as an extra assurance, but I would not worry about it.

Many things will trigger positive, such as remnants of poppy seed bagels and inspectors are trained to look for these and make a holistic determination as to the passenger's risk profile.

Is it likely that a scanner or swab or whatever those silver wand things are would pick up any trace of that stuff?

Both drug detecting machines and sniffer dogs operate within the physical realities of our world rather than in a magical CSI-like universe. Therefore unless your backpack still has a sealed compartment with hidden drugs, it is extremely unlikely any sensor would be able to pick up any traces after you've throughly washed it in a washing machine. Remember that pretty much anything contains a trace of drugs these days, so even if a machine is sensitive enough to figure out drugs have been in your possession at some point of time, it would likely be set to a higher threshold to avoid false positives from the majority of travelers.

Relax and enjoy your backpack.

They do have swabs and machines that can detect traces of hard drugs at Canadian airports, but they're going to be most likely checking on arrival (that may change after October 17, 2018) and you have what is likely a good enough story assuming there's nothing there to find.

You'd have to be very unlucky (or be doing something to stand out) to even get that treatment unless they're suspicious for some other reason. The only time I've seen them is on the Border Security TV show, whereas I've been swabbed for explosives a couple times and often have my carry on rifled through because of all the weird things I carry. People I know have had problems because of residue on their shoes and clothes (from various legitimate activities) that light up the explosives detectors.

In Montreal, as I went through the full body scan machine and my baggage and backpack through the baggage scan, something was set off. I then was patted down by a female agent, all my baggage was searched by a young male agent, each rolled-up article of clothing unrolled, all my camera equipment, each part of the baggage and articles.

Finally, when I asked they said it was probably a "trace" of some chemical, "Tetracyl or Tetraxyl" was found. When they asked where and at what job I worked and heard I was a nurse at a large health centre in Montreal, they said that must be it! (in psychiatry but I do give liquid long-acting injectable antipsychotics, so maybe that was it).

They also asked what medication I take... Fortunately, I arrived at the airport early but I was in shock after and barely got to my gate! It did not help that in the past the procedure of placing stuff in bins mixed up with others meant my laptop was picked up by someone else! So I was already anxious so maintain your cool!

Domestic travel:

Swabbing is primarily for explosive residue. With the changing laws, swabbing for drugs at the terminal entrance would not be practical.

International travel into Canada:

Perhaps nothing or you can be denied entry under the suspicion that you will use drugs in the country.

If you can find it, there is a TV show Border Patrol where you will see this scenario come up several times.

The traveler is always thoroughly searched, then the border officers will make a decision.