Coolant missing from 1996 plymouth voyager?

My 96 plymouth voyager has coolent dissapearing at a slow rate, it still heats up normally, the oil looks good and is not rising, there is nothing on the ground and I dont smell coolant from the heating core. I am going to pressure test the system some time soon when I can get it to the shop, I just want any ideas before I do it. oh and there is no white smoke. please help me=)