Same thing once happened to me at a Burger King. I was on duty, pulled into the parking lot in my cruiser, exited and went inside. There were only two customers seated in the restaurant.
As I walked to the counter I saw an employee run up, turn off the cash register and then run to the back. Although I thought it was odd I continued to the counter. I stood there for a minute but nobody came out. I called out and saw an employee look around a corner but nobody came out.
I stood there for a few more minutes and realized they were not going to serve me. As I was walking out one of the customers seated in the dining area realized what happened and said “man, that’s messed up.”
I didn’t make a complaint, I just left. Probably better not being served than to have them put ‘extra saliva’ on my order.