Vibrating ecm while vehicle is off

Here’s something that may be interesting. I was planting my hideakey spare key box under my truck and I heard vibrating coming from a black box. The truck had been off and parked for at least 8hrs. Any idea what could be on/running in the box, is this the ECM box? Is there a fan for cooling electronics in this box perhaps?
the location Of this box is under the bed towards the drivers side in front of the gas tank.
2013 tundra base model.

 

Normal. Not the ECM. Fuel vapors getting purged by the evap system.

 

That's the Broncos new logo ^^^^^^^^^^

 

Thanks for the quick response, that explains it perfectly.

 

Welcome from Colorado

 

I was going to say govt spy device. But we all know where your hidden key is now.

 

I already made a copy.......

 

The real problem was I locked myself out of my truck the other day and I just knew I had a hidden spare key. After looking furiously I couldn't find it and had to call the wife with a spare to unlock. This was maybe the second time in 20 years that I have locked myself out of my vehicle. So after driving home I decided to look again, well you can imagine how frustrating it was to finally see the spare key looking back at me. I use a magnet and duct tape to hide my spare keys, this one was camouflaged so well it looked like that sealant used between exterior and interior sheet metal components. So you can also imagine that finding this spare key and hearing the evap fume extractor and thinking "now what" made for a humble day.