Acid etches glass.
What kind of acid?
Gee, what would it be like to come to work and see this on your window? Nothing can remove it, just replace the glass. Freakin expensive.
Here is an article on Worksafe about the risks of using hydroflouric acid to tag. Watch your skin, your lungs, etching goes on for days. There are other products one can use. Check out art supply stores.
Is this vandalism?
Tags like this are everywhere but we sort of don't see them; they are on phone booths, mail boxes, subway windows, you name it.
Here is an article in the NY times dating from 2006 about acid tag-etching in the metro.
Paris is the place for street art: read it here on Bloomberg