Not just squirels, but all wild animals have 1000x better senses than us (all but the brain) so i am suprised that more people do not pay attention to whatever: be it squirels barking, other deer looking in a direction, blue jays or other birds acting up somewhere, etc. They WILL tell you if something is near if they sence it. Also, if the woods suddenly goes dead, then there may be another preditor around be it human, coyote, fox, hawk etc.