I've had that happen before...freaked me out a little, found it was nothing, but I know that if there was someone dangerous there, the girls would chew them up pretty bad.
Colby sets off the alarm if she hears even just a little squeak from outside....she starts barking her lil' head off, then Tippy joins in while May dances around like, "Wha? Whazoutthere? Hey guys, get outta da way!! I wanna see what's out dere!" And Colby gets so annoyed if you tell her to be quiet, she grumbles at you like "You can't make me shut up, you stupid human!"