A peace officer is sworn to uphold the law. This means they can be fired for indescretions "off the clock" and on the flip side it means if they are acting as second job security at Wal-Mart in uniform people are expected to do as they say. I don't know where the plainclothes fits in here, as I have personally told many a security guard to kiss my butt because they have no legal authority outside of private property protection.

Seems like a cop-become-guard not in cop uniform could be a tricky situation