Who does that?

We've been in our house for 4 months now. We've walked up the stairs countless times. Today was the first day we noticed the gum on the underside of the railing/banister. 4 pieces left behind by the previous owners. Ick. How did we not see it in 4 months? Better yet, who does that? I'm willing to cut them some slack and assume that maybe small children did it. However judging by the munge we've had to clean out of so many other places, I wouldn't be surprised if it wasn't children at all. It did chisel off quite easily, but again, ick!