I searched all toilets, but there were no keys. One of them contained a corpse though, but I couldn't open the door of the stall.
In The Men's stall you'll be able to open the door of the washroom and in one of them you'll find a opened toilet cover, inside the toilet you'll find the key.
but first you'll have to go to the kitchen's freezer to trigger a dialogue of lenny being in the freezer then you'll be able to find the key, click "E" to interact.