Car runs fine and only starts to heat up when idleing
for 10 min or more. Cools down when heater is on and
when driving. I noticed my reservoir for coolant was
empty and can see a very slow leak coming from under
the car. What could the problem be. Heater works good
and air conditioner also works well. I don't see any leaks
from visible hoses.
By "heating up" do you mean it takes a long time to get to normal operating temperature or it starts to run too hot? Low coolant will make the temperature go up at slower engine speeds.
If the leak is too small to find with a pressure test, you can add a small bottle of dark purple dye to the coolant, then search a few days later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Auto parts stores have the dye, and many of them rent or borrow tools. They should have the black light too.