They were around in public loos a year or so ago where I live but the Council authorities have come down on them, most are blocked up now - and I mean really blocked (steel plates and the like.) The cops were in cahoots. They had an eye out and prosecuted quite a few 'participants.' A flurry of articles in the newspapers. My only experience was at the Uni toilets when I was student. A few had holes. I assumed students were 'safe', i.e. randy, like me, after a few hours of library study, and in need of a quick fix. Public toilet glory holes, from what I occasionally saw of them, always struck me as plain dirty and slutty. Apart from that and the large risk of infection I'd never have been sure there wasn't a nutter on the other side.
onder: