i passed the same guy the other week on my sp.... tbh i was expecting him to be there - he was chatting to another plod on a marked bmw, gave me a nod as i bimbled past
just have a think to yourself where you would hang out if you was a plod.... if youre awt like me you're probably bob on
btw when you say he was in plain clothes - they have to be identifyable as plod if they are going to pull you over, so they have to have some police markings on their 'uniform'
the ones you have to watch out for are the unmarked bait bikes with 100% civvy clothed plod - these are spotters and will not pull you over, but will radio ahead and vid you and let the marked boys pull you in.
just beware of an unknown biker tagging onto your ride....