Thieving Sales

Someone is stealing sales from my entire team. I have no idea how, but it is happening. What I want to do is either stop this petty theft, or steal our sales back.... Any suggestions?

I think you need to be a bit more specific if you want advice.

Lets expand the story a bit.

You work at a used car dealer and your competition is across the street.
When you are working a customer, their phone rings, they look up and across the street and wave. You wonder why.
They say they want to sleep on it and will get back you.
You see them head to their car, drive around the block and pull into the dealer across the street.
You wonder, that's weird, someone with the same car just left here.
Those people drive away from the other lot in a new car.

DAMN.