Hi,the leaking rib theory makes perfect sense but when I checked the picture, the marks seem to have a copper tone to them. Could it be that you have gone over some already dry rusting solution again? I use a homemade solution which contains copper sulphate and it leaves a copper coloured stain if I accidentally re cover the evaporated area or handle with solution on my gloves.
I may be well off the mark as it could just be down to colour cast of the photo but if not, I hope this helps.

Rust never sleeps !