What was this truck-mounted widget?

[DaveC426913]

I saw a van downtown that had a spinny widget on top. (The van belonged to a company called Carmichael. It had extension ladders on its roof.)

Anyway, this widget looked like a black stereo knob but six inches in diameter - a circular base with a bar across it. It spun at several rotations per second, like the short-wave radar bar seen on boats.

Its rotational speed seemed to be correlated with engine speed but I'm not positive.

Anyway, my hypotheses:
- a wind-driven cooling fan for the interior
- a GPS tracker

[ EDIT ] Ah. There it is.

Ironic. I was actually Googling for pictures of van rooves so I could PhotoShop a pic for clarity. And this pic popped up...

 

[brewnog]

Air vent.

 

[FredGarvin]

I was going to venture a small vent fan.