Somebody who doesn't know what they are doing, sure. Somebody who does, and maybe even enjoy doing it, and the challenge of it, will surely know about it simply because they are maybe two dozens of types of smartphones out there. Android (including of course GrapheneOS) based ones, iOS ... then PinePhones, Volla, Purism and few others. So having a guide on how each one can have hidden partitions or booting mechanisms is tractable.
Point being that relying on a hidden trick (rather than encryption) is a very risky bet.
Coincidence but I just started to listen to 404 media podcast on tracking rare books being scanned in AI facilities thanks to just a cheap AirTag. I find this a good illustration of how easy it is to find something if you know how to look for it, and thus again why encryption is the only safe way.
Point being that relying on a hidden trick (rather than encryption) is a very risky bet.