I've been subject to very similar incidents while out harmlessly, boldly shooting in public but I'd like to relate a more hopeful story on the topic.
A few weeks ago I was out photographing, and after getting all set up in a difficult position I noticed that a Police car had pulled up across the street and the officer was eyeballing me intently looking none too friendly. I guess that he was trying to think of any reason why _anyone_ would want to take a picture of a small, picturesque, historical stone church on a beautiful Fall day with large dramatic clouds and colorful foliage in the background, taking his time with a 4x5 view camera on a tripod next to a busy roadway in broad daylight, making no effort to conceal himself or his actions... unless he were somehow plotting the downfall of Western civilization.
After a while he drove away without even bothering with the obligatory, routine photographer harrassment. Chalk one up to freedom to photograph without interference from the authorities.