In my case, everyone is supposed to be visibly wearing their company ID, on a necklace/lanyard or something like that. However even if challenged without one, "

In my case, everyone is supposed to be visibly wearing their company ID, on a necklace/lanyard or something like that. However even if challenged without one, "oh sorry, I left my ID at my desk" is likely to work. However everyone but brand-new employees is a least vaguely familiar to everyone else, so I'm not sure how easily a complete stranger would be able to move around.