Do you see what I see?

You know how some fire alarms spray ink on the person who pulls the alarm, so they can identify the culprit in case the alarm is pulled as a prank? That's what I thought of today when Max came into the room after coloring with his markers in the kitchen. He had blue marker all over his hands (to be expected), blue marker on the paper (obviously) and blue marker one other place, where it's not supposed to be...

I wonder what happened here?