Most supermarkets have bathrooms, and may have showers for employees.
I got the impression the sign was accessed from outdoors. That she didn’t actually have after hours access to the inside of the market.
There’s a somewhat standard Walgreen’s store design with a huge space above the entryway. It is all enclosed but it always seemed like it would be a cool place to put a little secret area there. Kinda like a bigger version of George Costanza’s under desk nap space.