I see this sort of argument thrown around a lot. I usually hold your opinion in high regard, and think that you usually manage to be big fan of Monzo and still see things objectively, but please allow me to express my strong disagreement with you on this one.
Firstly, anyone who says “trust me, I know what I’m doing. Don’t ask me any more questions” immediately looses my trust. (I know you are not Monzo, and that this is not what happened here.) I do not trust anyone with my data, just because they or someone else say I can. I scrutinize them, and then make my decision. To just say “you can trust them, simply by virtue of being a bank” is (pardon my french) rubbish. Every organisation makes mistakes. Every individual, working for any organisation can make mistakes. Do not trust someone just because they are [insert trust-inducing word here]. Everyone who wants your trust needs to earn it.
As for banks specifically: Anyone in the security business knows that “bank grade security” is usually an example of how not to do things. I’m just gonna give you three examples of this:
https://twitter.com/Floppy/status/914818418880245760
(That last one would be so funny, if it wasn’t such a tragedy.)
I would definitely trust the likes of Google more than my bank to protect my data from hackers. (That’s not to say that I’m not 100% aware that Google will voluntarily share a lot more data than my bank will, and probably knows a lot more about me than my bank does, but that’s another story.)
Now, maybe you say “legacy banks - Monzo is better”. My reply is “Maybe, but Monzo needs to prove this”. And quite frankly, between the fact that receipt images are available for anyone on the web without expiry date, even after deleting them (and that’s considered “good enough”), that Monzo allowed me to change my address while only asking for publicly available info to verify that I was I, and that Monzo refuses to provide a locking mechanism for their app because “reasons”, that doesn’t instill confidence in me that security is taken seriously.
Now, about the actual question: I also find this very weird. I was never asked by any other bank for source of funds. And that’s despite moving sums significantly larger than 3k between my and my wife’s current and savings accounts back and forth and in circles, to an extent that I always expected that the money laundering or fraud alarms would go off eventually.