I hear what you're saying Dave but believe it or not, I've seen rust do this before.
The cleanliness of the area prior to painting is critical. If the paint doesn't adhere fully condensation will begin to form and over time it takes on a capillary action forcing it's way under the paint along the path of least resistance..
Options
- Leave it alone and try to ignore it.........that would drive me buggy....

- Investigate further and carefully scrape with a razor scrapper to determine if in fact it's rust from within...
- Take it to a shop and they'll hit it with a sander and determine the corrective action.. :buffing: