That might be it then. If he could reach it. I had a beautiful autoflower growing that was picture perfect in preflower about a foot tall at around 3 weeks old a year or so ago. I used this organic fertilizer (which I highly recommend) named Big Blooms and Buds. The Nitogen fertilizer has...
Weird. Do you have a cat by chance? I've heard of cats nibbling on weed plants. If left outside it could have been anything. I would have to think that something nibbled off the top of your seedling.