It simply is that maternal does have a very small home range, and they know every tree and rock. She knew that lump in the tree didn't belong. Like if you looked out your window and saw a strange car in your driveway....you would be curious as to what was going on. She was investigating a change to her home environment. Had you turned out to be nothing more than something harmless, she would have taken note of that and not reacted the next time. Like the guy said in Jurassic Park...."They Remember!"