Never been to a Walmart specifically, but every store around here broadcasts a "closing the store in X minutes, please be outside of building by then" message on the loudspeakers a bunch of times, then the employees pack some things up and walk through the entire shop to check if everything is ok and the security personnel, being the last to leave, check all cameras before finally locking up.

This seems like plenty of due diligence for the store not to be liable is someone gets locked inside.