The dealer won't do anything about that unless the metal is perforated.
That's an interesting statement, and I hope you don't mind if I explore it a bit... I'm not defending or attacking either position, but just exploring them...
Any rust spot, if left alone, it eventually perforate the metal. While I'm not familiar with the metal used for the CX5, it doesn't seem unreasonable that the thinner metal, regardless of it being "high strength", would perforate sooner.
So, should the OP just wait a little longer for the metal to perforate and THEN bring it to the dealer when it's more of a problem? Or, could the argument be made that the rust spot is a clear sign of imminent perforation, and therefore should be fixed by the dealership?
I can't seem to find the mazda cx-5 warranty booklet online, so I'm not sure how they word things. (That's interesting. I can find all the manuals on the mazda website, but not the warranty book? hmm..)
Edit: so, went to my car and looked at the booklet in there. As long as the corrosion can't be proved to be due to something specifically excluded, and as long as the hole appears within the first 60 months of ownership, it should be covered. The question of imminent perforation is probably best left to lawyers.