Investigators, private detectives, spies, others who act for the government including telecommunications workers WHO wear baseball caps with video and recording devices secretly (or not so secretly attached). Generally they walk around looking totally disinterested in anything and rather gormless, a total giveaway!
I noticed a couple of guys walking up and down outside my house wearing baseball caps, then one of them started talking to the small microphone on the cap. Very strange behavior! Either he was a wacko or rather nosey.