overheating, ? Options

[jayi12-15psi]

Lately my engine has been running warm, it's not getting too hot, just almost to the top of the little temp. emblem(as opposed to being on the little wavy line). I replaced the radiator in august, and it started doing this a few months later, so I took it to a mechanic. The mechanic found a small leak when he pressure tested the system(it was a hose he tightened it, and filled the radiator back up with coolent). Then about a weak later it started doing it again. I seem to be losing a small amount of coolent, is it possible for coolent to somehow disapeer in a pressure tight system? Because I can't seem to find a leak.

Thanks for listening.

[jayi12-15psi]

well, how much loss in coolent would make a difference? For instance I add coolent everytime it starts heating up, which is about once a week of daily driving. Maybe every couple hundred miles. Each time I only add a little coolent, as in maybe a couple cups worth. So if the coolent is just a little low, will the temp register low? I've never replaced my thermostat, and my car has about 187k miles, I'm just trying to avoid a costly diagnostic.

thanks again guys.