I have one in my bathroom at home, although it’s a plastic rather than metal plate. In my case it was because when the pandemic started and everyone was on lockdown I determined that my central heating system was leaking, so unable to get someone round I cut a hole into the void where the pipe work runs between floors to see if the leak was in that area, which it wasn’t. I then covered the hole up with a plate as I don’t have plastering skills. I’m sure this wasn’t the reason in Birchhanger services public toilets, it was probably something to do with penises.