Too quiet

When a 17 month old is in the next room, and you can't hear his toys making noise, it's too late.

I should have been watching him closer. He often sits quietly reading books. I scooted the computer chair back and saw that he was drawing with a pen on the cardboard box that is his bookcase. I should have taken the pen away then. A few minutes later I checked again. He was happily drawing on the foam floor letters (and his arm) that we have on the living room floor. When his dad came home I said that DS1 had "marked his territory". All but 3 of the visible letters had marks on them, some just a few marks, some were very well "colored in". Seeing them makes me smile, but I should have stopped him sooner.