I have seen rust. Have a friend who commutes in die-hard winter conditions. He developed some surface rust on his stainless cog.

Also, some of the older cogs are not stainless. Look at the part number to see whether it begins XSE. XMN cogs are stainless. XSE ones are not, and are no longer made.

You might also shoot an email to Gates, or to your bicycle brand, and see whether something can be done.

If it's light rust, I might not even worry about it. My friend's was that way. Sounds like yours might be worse though.