I once aprehended someone at a RIAT acting supsiciously in the car park.
He was a coach spotter , which is even sadder than collecting a/c reg numbers...
I once caught a chap acting suspiciously around our antique Foden bowser one Saturday morning.
When I enquired what he was up to, it transpired he was a Bowser spotter.
He regaled me with the history of our bowser and how he collected them and that he owned a similar model to ours.
I left him to it.