something spooked my dog...

[X000BOSS]

i have a beautiful doberman and she is a fearless guard dog(so i thought!). Two nights ago she was barking in a strange way. i went to investigate and she came upto me and behaved very strangely, almost like she's turning to me for comfort. she kept staring at the far corner of the yard and growling with ears pricked up. she was barking at what ever was there but not in her usuall "i am the boss" way. we've had her for about three years and she's never been like that before. i could not see anything there and have no idea what spooked her.
thoughts that crossed my mind: person, no way. she'd try eat them. haha.
rodent, no way. she does eat them.
another dog, ummm no, she never backs off.

any ideas anyone? anyone seen their dog behave like this before?