I purchased my FN 510 on 05/06/2023 & Ive carried it every day since. Its seen very little actual range time, about once every 2-3 months. I clean & lube it after each range session. Never a hiccup with it.

However, recently I went to replace the battery for my RMR & discovered that under the factory provided RMR adapter plate, my frame has a considerable amount of surface rust! Its just surface rust, but this guns well under a year old & other than sweat from being EDC'ed, has never seen water. This is very disappointing in a $1100 pistol (hate to say it, but not a single one of my old ass Glocks has ever shown a spec of rust).

I emailed FN to file a warranty claim, but my trust in the pistol has definitely taken a hit. I kayak often & while the gun remains in a water tight fanny pouch, id like to know that it would fire should I need it to! Im a Floridian & often kayak through gator infested bodies of water. Sometimes these get very narrow & im inches away from them. I need to know my gun will work should I need it to.

Has anyone had a similar experience / issue with their FN? Ive heard reports of the 509 rusting early on too. Im hoping its not a common FN issue...

It is not rare for carry pistols to develop rust regardless of who made it.  I have seen Glocks rust and I have seen the most expected pistols not rust.  You say it has been exposed to sweat that is an interesting little thing to note for example..  I daily carry my pistol, I mow with it, I stuff it in gym shorts, go for walks with the wife and kid, and I sweat like a pig in the S. Texas summer heat but I never see rust on my pistols that is just me.  Now my best friend he sweats a lot in the summer heat and every single pistol he has ever carried develops rust from his sweat.  It comes down to everyone's sweat chemistry is different and he just has to do maintenance much more often than I do.  
One way you could fix this and prevent it in the future is apply a thin film of grease over to in a sense seal that area.  I know several people who have had their slides cut and opted not to have the slide refinished but just put a layer of grease down over the exposed metal.

I'll definitely be adding some grease or CLP before putting the RMR back. I appreciate that advice! I too suspect my sweat is to blame.

I would avoid liquid oils like CLP and stick with grease.  The CLP will just slowly weep out over time collecting dust where any excess grease will be pressed out then wiped out.  After everything is said in done the grease is not going to go anywhere from that point.

Ahh gotcha, that definitely makes sense. Thank you so much for the advice man!

Florida humidity, salt and acid PH from your sweat, will cause anything to rust especially if you're carrying it everyday. Dont be surprised if they deny your claim.

If by some miracle they honor it I would sell it, take the loss and get a Glock if that is the only brand you have had success with. Yes, it's sad....I have many pistols I love but would NEEEEEVER carry

So to make sure I understand...

You have EDC'd a pistol for nearly a year in a warm, hot and high humidity environment. You have taken it on bodies of water, and you are surprised to find a small amount of surface rust in an enclosed area of the pistol where you did not apply any rust preventatives?   And you somehow think a small amount of surface rust is going to effect the operation of the pistol and should be sent to FN as a warranty claim?

You sure are ASSuming alot. I never stated it would affect reliability. My $600 Glocks didnt rust, so why should my $1100 FN.

Stop trying to be a wise ass, youre not wise, just an ass.

Thats simply untrue. My M&P didnt rust. My CZ never rusted. Hell, my P320 (which i didnt like overall) never rusted. Nor did my Springifeld. None of my steel framed revolvers or 1911s have rusted other than 1 which I bought used & came with the rust (very little / surface).

If the gun functions properly, put a thin film of Eezox where it’s rusted, and move on with life.

Dip it in cosmoline if you’re losing sleep.