2005 Jeep Grand Cherokee Slow coolant leak

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.