This is some tagging that I can get behind!

Check the vid -

I wonder what would happen if anyone in one of those windows was looking out toward the source while it was firing? Could you damage someone's retina with that thing?

The green laser is really powerful - I've seen clips of similarly powered lasers cutting through electrical tape, popping balloons, etc, so yeah, that could do some damage. As far as the 'tag,' I think that they're just using a fairly standard projector.