Well that's easy enough - don't apply sneaky pricing when there's two people looking.

This arms race accelerates quickly. The question becomes stopping someone observing from a distance. It would have to be very tight to go unnoticed, and it seems likely to me that when detected it would quickly become a costly PR SNAFU, in addition to the cost of all the tech you need to deploy. I'd guess that grocers have little disposable capital anyway, given how low profits tend to be.