Only happened to me twice. Both at the same ground, coincidentally. First time was the home side pissy that they'd lost and were just being nobs about it and said they weren't going to pay. So I went home and reported it. Eventually got the match fee from the league and they had a fivefold fine on top. Wasn't going lose any sleep over £15, but damn right I'll report every single of detail about it.
Second time was a curious tale, took the match ball back to the changing room as was standard back then. Got showered, changed, came out and everyone else had buggered off. Phoned County on the Monday to let them know what had happened, just as a "now what?" as I still had the match ball, and didn't want to just be knocking on the home club secretary's door (couldn't get hold of him on the phone). I got the bigger bollocking for "stealing" the ball.