"Random" coolant squirt?

[Bigbunt]

Okay, what am I missing here? I've put about 1k miles on my '67 two door with b20 since going through the whole car (almost!) after a lengthy time off the road. All new coolant hoses, repaired leaking radiator neck and pressure tested radiator, new caps on radiator and overflow bottle and all seemed to be fine. This morning went out for coffee and got a whiff of coolant which came and went but I pulled over for a quick check. Found coolant splashed/squirted all on the right side of engine bay. Hard to determine source. Added less than a quart of coolant and drove home carefully. No overheating. Rinsed whole area with fresh water and let things dry out a little.
A couple of hours later, started it up and let it warm and pressurize and stared intently with a strong light at the whole area and no apparent leak at all. Did notice a little bubbling but no fluid leaking from the small vent hole on the back side of the overflow bottle neck at the pressure cap. One theory I have is that the cap released pressure and belched fluid out the vent? Could that happen and why would it is one question. The next question is what else might do this? Kind of confounds me...