they don't believe you're doing anything in there.
ha ha i still live at home, and my dad will yell to clean up the kitchen right when i take a shower.
It sucks!! But just a coincidence unless you are aware of someone stalking you or wanting to keep you uncomfortable!
Its a conspiracy johnnyboy55
They're hoping you'll answer the door with no clothes on or with your trousers around your ankles ;)
You've overlooked the vastly more important question: how do they KNOW you've just gone to the loo?