I'm not an expert on these things but I have done quite a bit of soldering though out my life. From what your describing it sounds like flux weeping through a small gap that didn't completely get soldered. Flux melts at a much lower temperature than solder does and it sounds like when you get the barrels warm the residual flux in melting and seeping from a small area the solder didn't take on. If this were a joint on a copper water pipe you would have a small leak at this joint.
You may be able to confirm this by taking a thin liquid like cleaning solvent and putting a drop were the leak is and seeing if it wicks into the rib joint. The other thing you could do is if there are small holes somewhere in the rib apply some low pressure compressed air and see if you get leakage in the concerned area.
Just my 2cents worth.
