Technically, given they are shared bathrooms for the entire building - yes, you're likely in the room.

That said, as a woman who has often had to use men's rooms in a reversed situation (i.e. 90% women staff, 10% men - who were almost NEVER in the office) - we did this too. We would open the men's room door, yell "Any men in here??? Woman coming in!" and then do so. And even put a post-it "woman occupied" on the door. But we all knew we were doing it.

I've spent some time in Europe where they have all gender bathrooms and honestly, it's pretty awesome. No one cares there. That said, the doors are flush to the frames and go nearly to the floor and such. Unlike our cheap ass crap here.