Random shutter sound
Your search for the origin of this sound may progress on 2 paths:
which application produces it and which sound is it.

Which application?

Here is an easy way to control if this sound is coming from
a standard screen capture.

Type the following command twice:

ls -lu /usr/bin/screencapture


First, whenever you want.
Next time, just after you heard the shutter sound.

This command will display you the time when this command was last run.

Which sound?

Quick identification

Here is a 1st attempt to be sure of which sound is used.
You can't try to recognize a sound by firing an application and trying
all the sound it can produce with its graphical interface.

The only practical approach is to use fast command lines just after
you heard your unsolicited sound.
Open a Terminal or xterm window and enter as is these 4 lines of
command defining short name functions to test 4 approaching sounds:

shutter() { afplay '/System/Library/Components/CoreAudio.component/Contents/Resources/CoreAudioAUUI.bundle/Contents/Resources/Grab.aif' ; }
lock() { afplay '/System/Library/Frameworks/SecurityInterface.framework/Versions/A/Resources/lockClosing.aif' ; }
unlock() { afplay '/System/Library/Frameworks/SecurityInterface.framework/Versions/A/Resources/lockOpening.aif' ; }
safe() { afplay '/System/Library/Components/CoreAudio.component/Contents/Resources/CoreAudioAUUI.bundle/Contents/Resources/Sticky Keys Locked.aif' ; }


On Mountain Lion, these sounds have moved. Then these functions have to be defined with:

shutter() { afplay '/System/Library/Components/CoreAudio.component/Contents/SharedSupport/SystemSounds/system/Grab.aif' ; }
lock() { afplay '/System/Library/Frameworks/SecurityInterface.framework/Versions/A/Resources/lockClosing.aif' ; }
unlock() { afplay '/System/Library/Frameworks/SecurityInterface.framework/Versions/A/Resources/lockOpening.aif' ; }
safe() { afplay '/System/Library/Components/CoreAudio.component/Contents/SharedSupport/SystemSounds/accessibility/Sticky Keys Locked.aif' ; }


Keep this window open, and as soon as you hear the unsolicited sound,
fire these four commands in turn to hear which one was played:

shutter
lock
unlock
safe


Next, to be sure, you can once more verify the access time of the identified sound file with the -lu options of ls. For example, you can
confirm that the lock sound was played with:

ls -lu '/System/Library/Frameworks/SecurityInterface.framework/Versions/A/Resources/lockClosing.aif'


Deep search

If this quick approach fails, here is a command to identify the file
which was used by the system to play a sound within the preceding hour
(-atime -1h):

find /Library /System/Library \( -type d \( -name "iTunes" -o -name "GarageBand" -o -name "Apple Loops" \) -prune \) -o \( \( -name "*.aif*" -o -name "*.wav*" -o -name "*.m4a*" \) -atime -1h -exec ls -luT {} \; \) 2&gt;/dev/null


If this command doesn't report anything, the next step will be to run
the same deep search within your HOME directory:

find ${HOME} \( -type d -name "iTunes" -prune \) -o \( \( -name "*.aif*" -o -name "*.wav*" -o -name "*.m4a*" \) -atime -1h -exec ls -luT {} \; \) 2&gt;/dev/null