Something similar happened to me about a year ago. I had just gotten my camera and my friend Tyler had just gotten a new lens. We both rode BMX at the time so we went to this abandoned Pac N Sac or whatever and I took some pictures of him doing wall-rides and other stuff and then afterwards we went behind the building and were looking around and there was a bunch of cool graffitti and old rusty fans and just cool stuff. We noticed 2 undercover expeditions on the side but we didnt think anything of it so were taking pictures and all of the sudden, 2 undercover cops jump out from the side of the building yelling and screaming at us to put our hands up. We put our hands up and both asked if there was a problem. Long story short, they were trying to crack down on graffitti and they basically were the biggest dicks in the world. They told us that we were spray painting on the walls and all kinds of stuff and after a while of getting fed up, I got a little attitude and just straight up told them "Look man, if i graffittied this wall, I wouldnt be out here in broad day light taking pictures of it with this $1000 camera sitting on a tripod when I see your undercover vehicles right there!" They eased up a little and one of them asked me to look through the photos. I said sure and showed him all the photos of Tyler riding and then the few that I had taken of the rusty fans and a/c units and stuff and they told us we could go but not to come around there anymore since it was private property and we could get in trouble for tresspassing.

Glad you didnt get in trouble though, seeing as how you didnt do anything wrong, haha.