It's not just us sheep in MA who think that open carry is unusual. I've lived in various places and worked in "free" states. OC just isn't that common. When people do see a gun on someone's hip who isn't clearly a LEO they can get concerned and will call the police.
"...open carry is regarded as a Right, and though any citizen may call the police if they observe someone carrying a gun openly, that citizen's 'annoyance and alarm' does not override the Right to carry openly."