Card skimmers at a supermarket chain – Inside job?

From the Mercury News (via ars technica):
“Further evaluation uncovered an extra computer board that had been placed inside the checkout machine, recording customers' financial information.”
So…how did a skimmer get inside the checkout machine?

I’ve never been in a Lucky Supermarket, but the places that I frequent that have self check-out lanes all have employees watching the lanes and presumably have security camera coverage.

These aren’t unattended ATM machines or gas pumps monitored by a harried convenience store cashier who has five other customers waiting to check out, so one would think that adding electronics to the inside of the checkout machine would be a detectable event.

The company CFO doesn’t think it’s an inside job though:
“Although the skimmers at Lucky's stores were apparently installed inside the checkout units, Ackerman said the company doesn't suspect an inside job”