Device Connect Disconnect Sound

I'm running Windows 7 with Norton Internet Security 2011.  I'm pretty diligent about keeping everything up to date.

 

For some time, I've noticed that I will out of the blue hear a device connect and/or disconnect sound when, in fact, no devices are being connected or disconnected from the computer.

 

Has anyone else experienced this or know of the cause?  Is this just a bug in Windows 7?  I'd like to know exact what triggers the event.