My A9 says it detects moisture but its not been near any liquid. I've been told it's a feature Samsung installs to scare you into buying a new device, is the correct?
I can't go into the technical ins and outs but it's a indication that your charging port needs attention. It happens to some Samsung units but out of the millions of phones that get shipped... a small proportion have this issue. In some cases its a genuine fault, in some cases its a software or hardware glitch and in other cases it is poor ownership from the owner.
That is a possibility. Or it can be a genuine fault... or water actually has got in. To get an accurate idea, book the phone into a authorised repair centre to be examined.