Stuck to the inside of the drum tucked down in the invisible corner where one of the three turny-handly-bits meet the inside rim by the door. Always.

That effect saved me money once! I always spin the drum round as part of checking for hidden socks, and in this case did so in the presence of the washing-machine repairman who had just proposed to replace an expensive bit. The unusual noise it made caused him to change his opinion to "it's totally broken, buy a new washing machine".