As the scope is left on, the Hogster will NUC less often. This happens with all thermals. There can be times where the Hogster needs to be calibrated but the software doesn't believe it is time yet. When in doubt, manually NUC the 35. It is a quick press of the button closest to you. You should hear the shutter click (engage). This is most likely the issue. For whatever reason, I would say the Super Hogster actually requires a manual NUC more often than I remember in comparison to the Hogster 35.
This will happen every time you use the Hogster 35 if in the Advanced Menu (Long press of the middle button) it is set to NO for auto-NUC. It is the option that looks like a camera shutter with an "A" in the middle. If set to NO, the scope will auto-NuC just for the first minute or so and then will not after that. It is up to the user to NUC when they please.
As for your question on humidity, if the humidity does get higher through a night, yes the image will be impacted, again on any thermal. From my experience, the Hogsters are about the least impacted of any thermal I have ever used.
You can set Ultra-Clear to yes in the Advanced Menu as well if the humidity gets really high. My best guess given you explaination is either it needs a Manual NUC or the scope is set to Manual-NUC.