PC beeps long after power down

All four of the PCs on my home wifi network (three laptops, one tower) were powered down (not sleep) last night.
Early this morning, over a about a fifteen minute period, I heard several single beeps. It is possible that each machine beeped once - I wasn't close enough to any of them to be sure. The tower, although powered off, was still plugged into an AC outlet.

During the time this was happening, one or possibly two of my Kindle units was in use. I'm not sure if these two communicate through the wifi or directly by Bluetooth.

Is someone accessing my PCs even though they appear to be shut down?
 
Oops - I was just told at least one of the units was in sleep mode. Maybe the beeps are incoming mail? I just changed notification settings and will see if that makes a difference.
 
Back