Fans running with computer unplugged?!?

Ok, I've got a weird gremlin going on here. Recently I noticed that my video card and power supply fans would continue running when I put the computer to sleep. Made a little noise, but they honestly spun pretty slow so I didn't worry about it. Then, today, I powered the computer off to swap a hard drive, and I noticed the fans were going when the computer was powered off. Weird. Then I unplugged the computer, expecting the fans to stop, but they didn't.

Huh?

Let me reiterate: My...

Fans running with computer unplugged?!?