[Android Central Question]

For weeks moisture detected message appears and now on a daily basis phone reboots by itself and moisture detected message appears.
Phone has not been water.

[belodion]

Nevertheless, there may be moisture, such as from humidity.

[Rukbat]

If the phone is in warm humid air, that humidity fills the phone. then if you take it outside, the cold air causes the humidity to condense inside the phone. That's water damage. Since there aren't many minerals in that kind of moisture, it takes a long time for the few that are there to eat through things in the phone and start causing problems. So the initial "wetting" could have happened last winter.