Yep I've had exactly the same thing. I think that the unit is susceptible to moisture as I was also in a downpour when this happened and took all my gear off to get at my multimeter only to find a nice healthy 14 + volts at the battery.
I just switched my unit off and carried on. When they 'dry' out they as normal. Whether or not its a good idea to sealk the back of them I'm not sure, I'm just gonna live with mine as I know it's foibles. The unit is after all just an indicator which does it's job.

Cheers Steve.

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