Often manufacturers would glue caps to the PCB. Although you'd like to think they did this for the sake of microphonics, the glue was there to hold the components in place while they were being soldered.
Sometimes with age and heat that glue kind of decomposes and spreads out over the PCB from under the cap. For all the world it looks like the cap is leaking, but it's not, it's just glue.
Not that glue is necessarily benign: in rare cases it becomes conductive and causes short-circuits.

