Compressor running even with car off ...

I have a 2006 Envoy Denali that has started to have problems with the suspension compressor. I noticed one day the compressor was running even though the engine was off, no key in ignition.
Lately it has caused the battery to drain.

My wife took it to her mechanic who simply replaced the battery but that wasn't the problem, it still runs the compressor overnight and it's only a matter of time before this battery dies too.
The mechanic had put a meter on the battery and noticed it drew juice sometimes causing the meter to fluctuate but couldn't determine what was drawing juice. Apparantly it wasn't the compressor because he heard nothing.

What could be causing the compressor to run even though the car/engine is off?

Thanks to any who can help.

Sounds like the relay is bad, should be in the under hood dist. box.

Sounds like the relay is bad, should be in the under hood dist. box.

Hi Maxwedge,
thanks for the reply.
I'm not a really great mechanic, but I know just enough to get me into trouble. lol
Could you please be a little more specific as to where I'd find that relay?

Thanks

Look in ther power center, under the hood, l/s of engine compt, shuuld have the info in your owners manual also.