I got pulled over by the police... License, Registration, Proof of Insurance

The other morning, on the way to work, I tried to get through an intersection but it appeared there was an accident under the light and at least 10 cars couldn't get through. To avoid it, I made a right turn into the parking of a drug store that went out of business nearly 3 years ago.
There was usually trash and overgrowth of shrubs in this place and more often than not, some homeless people had taken residence in the store front covering. I noticed there were 2 police cars side-by-side and they were talking. Having seen this hundreds of times, I didn't pay any attention to them.
Before I could exit the parking lot, I heard a siren an saw red and blue lights flashing in my rear mirror. I stopped. Rolling down my window a cop came up to my car asking if I knew why I was pulled over. My response was "I have no idea." He explained I was cutting through a parking lot but my reply explained about the blocked intersection and... the drug store was not in business anymore.
While he was checking my identification, the 2nd police car pulled alongside me and a woman officer got out. She mentioned it was 'private property' and I was trespassing. I said, there were no signs stating this and she pointed to a (really small) sign pasted on the side of the building column that read: Private Property - Keep Out.
I chuckled and said maybe they should have chained the entrance with the sign or posted it in a place where the warning was visible from the street and not AFTER someone enters the property. She nodded and somewhat agreed but said they were only giving me a warning and I shouldn't cut through the lot anymore.
The other officer came back with my ID and repeated it was only a warning. They had trouble with homeless people camping out in the lot leaving garbage and the landlord called on the help of the police to get them out.