Uncle Ralphy

While cleaning my wheels last weekend, I noticed the paint was bubbling around the bottom caliper bolt on the rear right-side caliper. (The bolt just above the brake fluid line entering the caliper.)There was also a small bit of fluid at the rim of the hole. The fluid in the reservoir is down a bit, but not below the bottom of the indicator arrow. I don't remember where the level was prior to my most recent trip. I cleaned the fluid off and have driven +/- 300 miles since then and have not found anymore fluid.
Has anyone experienced this before?? I'm thinking there has to be a crack in the casting to let the fluid enter the threaded portion of the casting?? If this is the case, why hasn't more fluid come out after driving/ braking during my 300 mile trip, some of it in mountainous terrain where braking was definitely required.
Cheers, Al