Being caught a second time is always my favorite scenario.
Last time it occurred to me, I had innocently walked into a supermarket to catch some cardboard to continue my hitch hiking adventure to nowhere. When I saw how plump and fertile the rubbish bin next to me was, I cracked faster than an egg and plunged my fingers into the supple sacks, ripping open their contents to my greedy eyes.
Upon plundering numerous vegetarian pies, a few boxed pizzas and some other processed foods, the manager came out, politely telling me to leave. I turned the corner, wrote my sign, and peaked around to do a quick in and out rescue of my goodies and the supervisor came out as I started searching the rest of the bags.
"D-g, what are you doing? You can't do that! I'll call the bolice!"
"Oh please no, I was just hungry, and had no idea this was private property. Please don't call the bolice, I'll leave immediately!"
And so she barked to a young lad to open up the gates, which were promptly opened and the lad told me whatever goodies in my arms should keep me right, all the while laughing his head off.
Overall, I try to keep a cordial relationship and keeping cool is ideal. Its their territory, which is how I see it. Entering it is usually going to bubble a reaction, which will generally be negative as the dumpster diver is always seen as a substandard person rummaging through a rubbish bin.
My major concern has been to catch any diseases or possibly get cut or hurt while doing it. A few weeks ago my hands were chewed up by tin trays as I passed them through a cheesecake shop's bin. It was worth it, but my finger got a slight infection that made playing the guitar a mild misery.