I've never received on any of my paper bills marks that cashiers make on bills when checking them with the counterfeit bill detector pen. Usually when they use that on my bills when accepting them, it leaves a slight mark on the bill. So, does that mark go away after a little while? I think it does go away because, as I've mentioned, I've never to date received paper currency with marks from those counterfeit bill detector pens already on it.
That mark will remain and it’s not a full proof method of detecting. Bills have been bleached clean and copied at a higher denomination, then spent.
All the counterfeit pen does is show if it’s government paper. Best to look for the embedded thread on the bills. They are in $5.00 and up. Just not in the one dollar bills.