1987 300SDL

I've been checking the level of the coolant in the reservoir each time I'm away from the car for 5 hours or more. I've noticed that sometimes it's full up to the top line, then another time it will be an inch or a half-inch low, and then the next time it will be back up to the line. There's always some pressure in the system even if cold overnight, so I remove the filler cap to relieve the pressure before I start the car each time. This of course raises the level of the coolant in the reservoir, but there still is as much as an inch variation in level at different times, even after opening the cap. I'm not adding any fluid and I can't find any leaks.

It doesn't seem to be a big problem, but does anyone know why this behaves the way it does. Obviously, I have a bit of free time today.

thanks
Bruce S