I believe there is the liklihood that semen IS showing up in "pre-cum" from time to time.

This is perfectly normal.

I'm reminded of much earlier in life when we were taught in (VERY limited!) sex-ed in high school that guys needed to be very careful in having sex. Girls could get pregnant from "pre-emisison" fluids before an ejaculation even occurs. "Don't always trust merely pulling out before climaxing!" in other words.

The notion of some "air bubbles" (noted earlier) could have some merit as well.

Main thing here is I see nothing to be alarmed about with what's being described. All should be well.