Once you reach the boss floor, eat any of the walls south of the room you spawned in. This should open up a secret room containing a hole leading to the first endless floor.
I also tried it multiple times.. so to verify: is it the little room you spawn in? or is it the room with the grid? the room with all the food or the room where the boss burger spawns? I tried exactly as you said the room where I spawned in and it shows no signs of a secret floor/hole?