I've seen what you experienced, Marv. (Notice, I didn't say it happened to me...) I had a similar,
but slightly different theory. I just thought that the warmth of the wick tube was maybe enough
to vaporize some of the alcohol in the burner, and since it expands when it vaporizes, it pushes
liquid out.
I may be soaking wet!
Anyway, this particular one apparently has enough venting just past the loose threads on the cap. As
I say, there is no gasket in there. I would drill a pin hole in it if it were a cap that sealed on tight.
Something I may not have mentioned earlier in the thread; The filler tube goes about half way down
into the reservoir, preventing the contents from sloshing out the top when it's moved or jostled.
Dean