A Tesco store manager has a lot of responsibility and is likely a well-paid, long-running member of staff. If they investigated something like this then I'd be surprised if head office looked upon it negatively.
Having found credit card leaks in databases I can assure you that all but the most top level people would try to sweep this under the rug. I couldn't push my find up the chain until the C-level came to visit.
I was just going to chime in a with a similar comment.
I'd love to know if a friendly Tesco store manager would get in trouble for helping to run an experiment with this though. If head office got wind - or were informed by the manager - I'd presume they'd either go mad and threaten/fire the manage in question, or congratulate them. I couldn't predict it either way.