I have come across many squatter dwellings that are either still lived in or have recently been lived in. Mostly I try to keep out of those rooms which are being inhabited, as in theory it is their home and I wouldn't like to intrude.
I have once met a group of squatters in an old car dealership, they were really nice people and even asked if we would help them decorate, we politely declined and asked their permission to come in and take photos, it was granted.
I have recently come across a home believed to be derelict, after several visits to knock with no answer, we walked around the back to find the window next to the back door smashed.. 2 weeks later we went back, knocked again, and then walked around the back once more... the window was still smashed. As one of us went to step through the window someone appeared the other side shouting, needless to say we ran scared, within seconds we were laughing but it wasn't a comfortable experience at the time.