Was this not answered on another thread recently? It's the gas being absorbed into the beer.
You can check for leaks by spraying soapy water at the top of your keg.
I did Indeed misread. Is there any reason why you're disconnecting from your keg?
Drop in temp overnight could account for the reduction in pressure.
If you've checked all your connections and there is no sign of a leak, then I wouldn't worry about it.
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