Cap is not the culprit.
Pressurized the system and saw a hose spitting.
Went to get a better look today with the car up on ramps and the belly pan off and can see water pump has been dripping (but maybe not on the ground..belly pan catching the drippings). Pressurized system and yep..dripping from water pump. Once again, a bit dumbfounded on seemingly premature water pump failure but this is accepted as "normal" in the community. Basically, once the car hits 4-5 years and 40-50k miles, the water pump can be on borrowed time.
Ordered new water pump and serpentine belt. Thermostat housing needs to come off to access water pump, so ordered a lower-temp-than-stock thermostat and housing.
Looks like a 4 hour job or so. Not sure if I'll tackle it myself or not.