Front subframe rust?

I removed the skid guards for the first time on my 2016 off road 4x4 and noticed that my front subframe has a very light layer of surface rust. I used an old tooth brush and some wd40 to remove as much as possible. What could have caused the surface rust? I live on the coast in California, salt is not an issue. Could the ac drain tube have caused this? Has anyone else noticed the same? Is it a known issue?

 

If you live even sort of close to a coast, salt is an issue. It's in the air.

 

Sorry that's just to funny...live on the coast and salt is not an issue!

 

Huhhhhh????

G.

 

That freeeze ocean breeze will produce surface rust to pretty much anything over time. Just scrub it off every once in a while and you’ll be fine

 

I'm about 15 miles from the coast, we do see see a decent amount of ocean fog from time to time.