To answer your question, it was likely a counterfeit detecting pen[0] that only leaves a visible mark on counterfeit notes. I generally only see them used on $1

To answer your question, it was likely a counterfeit detecting pen[0] that only leaves a visible mark on counterfeit notes. I generally only see them used on $100 bills; rarely on anything smaller. [0]: http://money.howstuffworks.com/question212.htm http://money.howstuffworks.com/question212.htm