In one job I spent most of my time on other companies sites surrounded by their employees, often with another colleague at the other end of the room. We so often had to pass passwords that a developer came up with a shared code based on song titles that we could whistle to each other without anyone realising.
It was obviously a flawed system given not everyone could whistle and didn't share common songs always, but I'd just moved there from a council where the way to find out the sys admin password was to shout across a crowded room, "What's the password?" and they'd shout it back.
To this day all my unimportant pass phrases can be whistled, just out of nostalgia. I think the tune is an aide-mémoire.