Domestic Fridge/Freezer warning "bleeps"

We had a strange occurrence last night. We have a 14 mph old Swift Conqueror which has a tall combined Dometic fridge/freezer. Around 12:30 last night it started to "bleep", usually a sign that the fridge door has been left open or when there is no power to the unit. We had set it on mains supply when we arrived on site yesterday, after it having worked fine in the previous weeks on either mains or gas. For some reason it had switched over to gas which was not switched on, although there appeared to have been no power cut, the timer on the microwave showing the correct time. I reset it but it persisted to bleep occasionally so I switched both the fridge and the supply to the van off and back on again, resetting everything but we had occasional bleeps throughout the night, resulting in a disturbed sleep. On rising at 7:30 this morning I reset the fridge on mains and power level 3 and have had no problems at all through the day while we have been in the van. Fingers crossed that the problem is sorted but has anyone any idea what the cause of the problem might have been?