I remember about 18 years ago flying for an airline that mandated uniforms in the sim and we were in the coffee room during a break at the sim centre in the USA. It was two captains being checked together with the TRE. An American pilot walked in and jokingly shielded his eyes from the glare from the 24 gold bars that were 'dazzling' him!
An airline that I flew for a while before that only required uniforms for passenger-carrying flights. I read a delightfully entertaining article by a pilot who had arrived in a jet airliner on stand at one of London's big airports and the two of them needed to be back in the crew-room quite soon so they left the aeroplane and 'legged it' across the apron in jeans and T-shirts with only their headsets in hand (before the days of tight rules on yellow jacket and locked up aeroplanes) and he glanced up to see astounded looks from passengers looking down from the terminal having seen a jet airliner shut down on stand then two 'shady' characters run away from it! He was one of the Best of pilots too!!