I think most of the time when I worked in retail we knew we didn't have something in stock but we'd have a look in the back to make the customers go away.
Oh no! I can't believe what you are telling me! Did your boss ever get upset when you pretended to look in the back?
No, he never cared.
It's unprofessional to pretend that you're looking for something when you know you don't have it. You should treat customers with respect, not lie to them about what you're doing.
I did it because I'm a bitch and I wanted to piss people off.
Customers count on you for help with important things so it is a shame you play tricks like that.