How seriously should I take a CBP interview where I was told I have a red flag and could only stay for 30 days?

Immigration's job is making sure visitors to this country don't overstay, don't seek employment without the proper visa, don't rely on social services, don't commit crimes, and aren't being trafficked.

He surely wanted to talk to your former employer to avoid a snafu that would cause trouble later, if it turned out you were likely to be returning to her employ. You are an immigration novice and she is probably an immigrant-employer novice, and it's cheap to fix now, and a nightmare later. Also if you didn't mention the boyfriend, this left a big hole in your story that it's his job to fill.

Question 1: Deception

They may state a narrow reason when asking for your phone, but it's legal for them to lie to you. Actually, they check the phone for everything they can. Based on what he found in your phone, something clearly made him suspect deception, since he warned you about deception after he returned with the phone. Probably, you did one of these things, and he caught it:

• you said you didn't have the au pair lady's phone number, yet he found it in your phone. He may see this as minor if it was buried deep.
• you failed to mention your boyfriend as a reason for your visit. That goes very sharply to "overstaying".
• you failed to mention your intent to seek other employment, which is a huge no-no on this visa, and you are expected to know that! "Free" employment counts - in fact it opens up more cans of worms about trafficking and tax evasion.

The penalty for deception is a lifetime ban from entering the US. Probably, they felt that whatever they saw didn't deserve that serious a response, but you've been served notice now.

Question 2: what's 30 days all about

It happens all the time that you say "I'll be staying 5 days" on the entry interview, and then they stamp your visa for 90 days or 180 days or whatever their standard stamp is. However, that is not an invitation. Next entry, they'll compare what you said (which is logged) to what you did ... and judge your trustworthiness by that. So if he made you promise 30 days, better keep your word.

Further, this person is an immigration expert and he is giving you advice. He is saying that if you leave after 30 days, you'll likely be admitted on your next visit. If you stay the full 88 days, it will create the appearance of "living in the US via repeated visits" and you would likely be refused the next time. That is good advice, and you should heed it.

Question 3: Deportation? Really?

Can you be deported? Not likely that CBP would hunt you down. But a chance encounter with police, or with a CBP patrol within the 100 mile border area, it's a possibility. But these aren't likely. Most likely the consequence is being refused entry next time.

How would they know?

Attaching information to a particular human is pretty much their thing. They certainly have a right to videotape - it is their house and you consented to enter, and I'll bet there's a sign. Personally I think taping is too balky/awkward to effectively make use of at the immigration stand, so I suspect they rely on written notes. I.E. They have a large database, probably akin to a sales contact DB, or customer service DB. A platform such as SalesForce comes to mind. They probably logged something like

stated intent of 84 day stay to see boyfriend John Smith (see 6/19 entry). Search of phone reveals possible intent to work in childcare for friends at no charge. WARNED DECEPTION and advised not to seek employ. Claims no longer working as au pair, unable to confirm. Means not checked. Admitted on VWP, advised to stay 30 days max.

So next time you sidle up to the bar in January 2020, they go

"Well, I see you left on Dec 1. It really looks like you're trying to live in the US through repeated visits. And I think this is something the last officer warned you about, am I right? We don't have a 90-of-180 rule here - our rule is we know it when we see it. Tell your boyfriend it's his turn to visit Norway. I must refuse you today.

"Also, regrettably, since you did not listen to the last officer's advice and you pushed your luck, you now have a refusal. That means you are ineligible for VWP. Now, you must apply for a visa. You might try applying for a visit in May."

Further to @Hilmar’s answer, and after update from OP confirming duration of stay per i-94 is 90 days, IMHO the answer to the question is ‘yes, you should take this landing interview seriously’.

A requirement of US law means they start temporary visitor admission decisions from a presumption of immigrant intent. Although your Norwegian citizenship means you can avail yourself of the Visa Waiver Programme, your personal profile is probably risky from an Immigration officer’s point of view. You are young with evidently few ties to home, you have funds available, you have a boyfriend in the US, and you’re building up a history of repeated/frequent visits. So my advice is to educate yourself about the rules (if you’ve not done so already) and make sure you stick to them scrupulously. Maybe think about not staying the full 90 days this trip, and waiting longer between visits.

First, check your admission stamp or record and verify the date. Just because the officer said "1 month", doesn't mean that they only admitted you for one month. There is a depressingly large number of CBP officers that simply get a kick out of scaring and confusing travelers and just want to see you squirm and grovel. It's entirely possible that you got admitted for 90 days, so check the actual document. If that's the case you can pretty much ignore anything the CPB officer said.

If you only got admitted for 30 days, you MUST leave on time. If you overstay, your chances of ever getting back in the US are almost zero for a long long time.

It's possible that CBP officer suspected you want to stay here since you already had a "work like" stay earlier this year and are back soon for an unusually long period of time. They would also take your country of citizenship into account (which you don't mention, so I can't comment on it).

Unfortunately, the rules are very murky and CBP officers have broad leeway and discretion, so it's perfectly within their rights to cut your stay short or send you back for any reason or no reason at all.

If you actually got a reduced stay, the best you can do at this point, is to play by the rules for now. It may take 2 or 3 more trips to get things back to normal. Make sure you keep proper records of ALL trips to the US and bring them next time you travel. It's perfectly fine to show these records to the CBP officers when asked and say something like "I know I got flagged during a recent immigration although I have no idea why. I've always been very diligent on following the rules and here are my records. Could you please advise my what I need to do to become a normal traveler again?"

If none of that helps, you may need professional advice.

Something that other Answers miss is: talk to the agency and see what exactly they say.

As much as we all want to rely on online media, talking to an actual person seems to be the most reliable way to answer your questions right now. Get the number from their website and actually talk to them. Ask them your questions and get answers directly "from the horses mouth" (from the source of knowledge). They should be able to answer any and all questions you have.

We can only guess, as we aren't border agents. I've talked to plenty of "barracks/locker room lawyers" only to find out they were 100% wrong when I did my own research, sometimes including talking to a real subject matter expert. Just because some people have a certain experience or "deduce" things by way of other's experience or lack of fully understanding what they read about the rules, it doesn't mean they actually know what they are talking about. They might, but the only way you truly know what is real is to ask someone that should actually know.

The border agent you talked to may have been trying to scare you, or they may have paperwork in progress to bring you from the "standard 90 days" to a lesser amount. Whether they have paperwork in progress, it might get overridden by their supervisor or whoever needs to review the case or it's just taking longer than usual to process so your online status isn't reflecting it yet. You won't know until you talk to them.