Where’s my water going?

[busmania]

I’m nervous about this issue. It seems I am having to add to the radiator every time I drive it or it overheats (or every few drives). I only have about 150 miles on it and it’s running great but if I drive a few drives, it gets hot and I add water and it’s fine. I’m nervous about where this water is going! My oil level is not changing, I see no water on my garage floor. Is it possible there’s a small leak allowing it to get into combustion chamber as I’m driving? I did the bubble test (remove radiator cap while running and look for bubbles) and I’m not seeing any bubbles. Thoughts?