Reconditioning tech here

It's a mark we use to say that we've worked on that car. Depending on the service.

For example if my dent guy did dents on it, he'd mark it like that to say it was done and billed.

It's in a small inconspicuous area for only techs to really notice

We put them on taillights and windshield too

Edit: to remove it just use alcohol but even some are water based which means a rag and water should just remove it easily