The portal is only there when the wall to the store is opening or closing. There are no doors behind the counter.

"Err, do you sell chicken here?"
The man put a bit more menace in his whiny nasal tone as he said, "Read the menu, ma'am."
I read the menu. They indeed did sell chicken and in several varieties. I considered ordering a soda and some food, but then thought straight for long enough to realize that I should wait till my stomach settled to eat. I'm pretty sure there is some rule about waiting half an hour before you try eating after nearly falling down a bottomless pit. Plus I had been feeling rather nauseous from all those teleporters.
Trying to seize a simpler avenue of escape, I reopened conversation with the man at the counter.
"Where does that portal in the wall actually lead?"
He snorts. "It just leads to a pocket dimesion that links all restaurants everywhere united under the Chic-Fil-A banner. It makes relocating easier and closing time actually mean something to thieves."