@mattb: Please can you try swiping up from your Home screen and tapping on the Clock app > Select the Alarm tab at the bottom of the screen, and check to see if any alarms are active here that match up with the unexpected alarm you're hearing?
If not then, next time you hear the alarm, swipe up from your Home screen and head to Settings > Notifications > Recently sent. If you see an app listed here that corresponds with the recent alarm sound, then please tap on it and adjust the notification options accordingly. Alternatively, open the app itself and check the settings to remove any unwanted alarms. Let me know how you get on.
