I can't say I routinely carry around $100 bills. However, if I were to, I think I would notice if one was missing. I do remember once mistakenly giving a parking lot shuttle driver a $20 bill instead of a $1 bill tip. It was an early morning flight and I wan't paying much attention. I found his extreme exuberance at my tip a bit perplexing at the time. It was only later that I realized I was missing a twenty and understood what had happened.