Thought since Adam didn't write this one up that I would. Adam and I were spotting at Cargo plaza from the UPS parking lot since the Air India lot was closed off (actually I think this was better since we were further back from the approach we avoided belly-shots). We were pretty discrete, shooting from my Jeep with the windows down and we saw other "spotters" without cameras come and go. We made it a long time, probably 2+ hours without being bothered when as the light was finally going, the PAPD pulled up. From reading prior experiences with PAPD, I had expected a fairly unfriendly encounter and to be questioned rigorously. To my surprise, the officer was pretty friendly.
Although a bit stern in tone, he was very understanding of the planespotting hobby and acknowledged that he knew what we were up to and weren't the first ones. He made no mention of photography being "illegal" or anything like that, contrary to the other Gestapo-like tactics I've read about. He only told us that we were raising suspicion having cameras and he got called. He did mention that he knew about "the website we frequent", which was encouraging. I made it clear to him that the last thing we spotters want to do is complicate their job even more. He told us something along the lines of "I can't force you to leave, but it would probably be best for all if you did", after which we said goodbye and went on our way.
Overall, a surprisingly friendly encounter, and I'd like to really complement the PAPD on how they handled the situation. It would seem they might actually be getting used to us.