[Brownie]

Without actually seeing it it's hard to say but my my guess would be that you're seeing oil.

 

[Brownie]

Keep HK's away from water and bright light!

Or is that Gremlins ?

I keep throwing water on them. They don't multiply......



As far as this discoloration issue....
If you can't see a problem with the back side of the front sight then you're good to go. Best way to handle the finish problem it with a holster. A few hundred presentations and you're finish irregularity will buff right out.