That's not entirely correct. The paper that US Currency is printed on is a linen-cotton blend, but it's still considered a "paper", even though it doesn't come from trees.

There was a fascinating article about this on Planet Money a few weeks back, including an interview with the head of Crane's, the company that provides the source paper to the US Mint.

Our old $100 note wasn't either. It was the colour scheme that was the problem. Nevertheless, if it's still common to use a pen to determine fake notes, your technology is miles behind.