Hiding in the toilet is well known by Guards, so I doubt anyone would get away with that anymore! If a Guard walks past the loos whilst doing a ticket check, and sees them occupied, they could just wait for the person to come out or get a colleague to keep an eye. If the person doesn't come out after a reasonable time (10min?) then the guard can unlock the toilet and see if the person is ok
I saw this when I was off duty on SWT, a couple went into the Loos together and closed the door, of course this is suspicious because either they are fare evading or doing some sort of hanky panky
So I found the Guard and he was happy to unlock the toilets to have a word, turns out neither had tickets and wanted to evade the fare for just 1 stop (Egham to Virginia Water!) End result was that plain clothes BTP took the couple off the train at Virginia Water and delt with them, hopefully a lesson learned!