Auburn
A dirt that comes from nowhere is almost certainly rust. I have run corrosion tests for about 4 years now testing the most popular lubes, greases, and waxes. I can tell you RemOil is one of the worst oils I've ever tested and it usually looks similar to the "control" piece which has NO oil or preservative on it by the end of the test. You would have to be diligent with Rem Oil to rely on it in some environments.
RemOil is by their own website MSDS 50% mineral spirits. This is why it evaporates so quickly and cleans so well. I spent the first 10 years of my shooting career using RemOil and I thought rust was a way of life with guns, until I tried something that worked much better. I'm sure some people will chime in about how much they like Rem OIl and never had a problem with it, but all things being equal Rem Oil will be outperformed by almost any other oil available. Even engine oil or Hoppes #9 will stave off rust noticeably better the RemOil. I'd get the VCI tabs or cards and put them in the cabinet too. A cheap digital thermometer that reads humidity is a big plus too. Keep it under 50% to be safe.
Products I've tested of with excellent rust proofing capabilities are: Corrosion X, Eezox, BF CLP, Outers Gun Oil, Gunslick Metal Seal, Gun Grease etc.