I know I've seen this term on another website, I just can't remember for the life of me which one...so if anyone on here coined this phrase, please post a reply so you can get the due credit deserved.
A Credibility Prop is basically an object or article of clothing one has that reduces suspicion towards them. This can include things as a clip-board, being "dressed for the part", carrying a breifcase, wearing construction gear, etc...
For example: Trying to get into off-limit areas in a mall, dressing as a would-be employee (dress pants, button-up shirt, tie, no jacket) would be a good credibility prop.
Credit to this term belongs to Ninjalicious, I was browsing his site, infitration.org, and I came across this. So, please give all due credit to Ninj. Thanks!