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>/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>/dev/null

Could it be the sound a banner ad is playing? Try an adblocker to experiment. For example, if using Firefox, you can setup http://adblock.mozdev.org and see of you notice the sound still occurring or not.

I had a weird noise once that sound like a system sound, but eventually discovered that it was an ad that was appearing on alot of sites I visited.

Another possibility: If you use the Keychain Access program and unlock a keychain, it will automatically relock after a period of time, probably 15 minutes, and make this sound.

(Background: I was taken aback by this sound and began to search the web for this "shutter sound" phenomenon. Using @daniel Azuelos' excellent answer, I was able to confirm that what I heard was "lock" and put two and two together.)