Said by Patrick in the Spongebob episode "Hall monitor" Wee woo expresses great emotion, in Patrick's case it is fear. It is also the sound that police officers make when they do not have sirens. Or like Patrick are scared and say it to remind themselves that they are tough police officers :D
Wee woo is the sound a siren makes. It is used in jest, to make fun of police cars, fire engines, ambulances, anything with a siren, really. Popularized by short films.
Jay: Haha, the principal's about to bust those two stoners!