This advice is for a guy who works in the same building as me but it’s useful enough that I thought I’d share in case anybody else finds themselves in a similar situation.
The door has a little metal bit on the inside.
Turning this little bit of metal clockwise has two functions.
Firstly on the outside of the door a little window goes from white to red.
Secondly it stops the door from being opened.
And that should save you having to apologise to people when you’re having a piss in a public building.