yeah, "squatter's rights" is more of a figure of speech than a legal thing in the u.s. where squatters basically have no rights. i used to be pretty up on this but i think it's changed (for the worse) in the past decade when squatting has become a lot more common.
first basic point - in theory - if the house is in foreclosure, that means presumably someone will have to buy it before kicking you out, which may take a while or may not even happen? the note might just be a formality (they don't know you're there) or even a fake (another squatter trying to take your plac). can you post a picture or anything?
adverse possession: generally the idea is if you can establish that you are a *resident*, i.e. you have been living there openly, then it is a (civil) question of tenancy not a (criminal) one of trespassing. as we know, cops are dicks and treat poor people like shit so this may or may not help depending on (as mentioned) how you can document your residency, what your local laws are and how capable you are of making legal move about it. adverse possession statutes often require that you can show that you tried to inform the legal owner and either couldn't or got no response, which is part of what makes it tough to establish in the states. and again, i heard some places have streamlined the eviction process for squatters since the foreclosure crisis started....good luck....