If it is loosing coolant over and over there are two possibilites;

1. You have a small leak that you haven't found, I had one on a 94 teg that would only leak when hot and under pressure. Never had a puddle or other signs, I just let it run for a half hour or so (in Phoenix) and went over each hose until I found in on the underside of the C shaped hose on the back of the head.

2. You have a seeping head gasket and it is sucking your coolant into the cylinder slowly. If this is the case you need to keep a good eye on the temp gauge and if it goes above half expect to do a head gasket replacement.