Random and brief fan spin up I occasionally leave my Mac mini on but with its screens off, the way my power settings is  set.. the Mac should go into sleep mode after 15 minutes of inactivity.

I noticed tonight that this did not happen, and randomly an hour later the Macs fan started to spin up aggressively for a very brief period (5 seconds).

Could anyone tell me what this could be? Or is there a log that I can go through to see what was happening?

EDIT (as suggest by @AnonymousAppDev)

    23/01/2014 01:30:46 GMT  
Assertion status system-wide:
   BackgroundTask                 0
   PreventDiskIdle                0
   ApplePushServiceTask           0
   UserIsActive                   1
   PreventUserIdleDisplaySleep    0
   InteractivePushServiceTask     0
   PreventSystemSleep             1
   ExternalMedia                  0
   PreventUserIdleSystemSleep     1
   NetworkClientActive            0
Listed by owning process:
   pid 182(coreaudiod): [0x000000010000178c] 00:00:57 NoIdleSleepAssertion named: "com.apple.audio.'AppleUSBAudioEngine:Bose Corporation:Bose USB Audio:14200000:1'.noidlesleep" 
   pid 1063(VLC): [0x000000010000178d] 00:00:57 NoIdleSleepAssertion named: "VLC media playback" 
   pid 72(hidd): [0x0000000a0000178f] 00:00:43 UserIsActive named: "com.apple.iohideventsystem.queue.tickle" 
    Timeout will fire in 557 secs Action=TimeoutActionRelease
   pid 1264(screensharingd): [0x000000080000175c] 00:02:50 PreventSystemSleep named: "Remote user is connected" 
Kernel Assertions: 0x10c=USB,BT-HID,MAGICWAKE
   id=500  level=255 0x4=USB mod=22/01/2014 20:59 description=XHC1 owner=AppleUSBXHCI
   id=502  level=255 0x4=USB mod=22/01/2014 20:54 description=EHC1 owner=AppleUSBEHCI
   id=504  level=255 0x8=BT-HID mod=01/01/1970 01:00 description=com.apple.driver.IOBluetoothHIDDriver owner=BNBMouseDevice
   id=505  level=255 0x100=MAGICWAKE mod=23/01/2014 01:41 description=en0 owner=en0


I suspect it has something to di with either VLC or my USB Speakers?
First of all, we need to see if something is preventing your Mac from sleeping.


Open Terminal.app
Type in pmset -g assertions


What you are looking for is these lines:


  PreventUserIdleDisplaySleep
     PreventSystemSleep
     PreventUserIdleSystemSleep
     InternalPreventDisplaySleep 


and 


  Listed by owning process:
    pid xx(nameofdaemon): [0x000000090000012c] 01:45:46 BlahBlah named: "com.randomnam" 


Copy and paste all of that into your question.

Next, try resetting the SMC (System Management Controller)

Taken from Apple Support


Shut down the computer.
Unplug the computer's power cord.
Wait fifteen seconds.
Attach the computer's power cord.
Wait five seconds, then press the power button to turn on the computer.


Finally, check the Console Logs


Open Console.app
Click in the search box and type in sleep
Find the entries that match the time your Mac was sleeping
Copy and paste it in your answer


The first and third fixes weren't really fixes. I just hope that the data you paste will help find the cause.