1997 Pontiac Transport engine coolant slowly disappears

I have a 1997 pontiac transport/montana with a 3.4 litre and it has 197000 kms on it, the engine coolant is slowly disappearing, i fond no viisible leaks, nothing in the oil either, the engine does not run hot, it stays normal temperature and after about 200kms i need to add about 30-50ml of coolant. what would be the suspect problem and how to fix it easily with low cost.in response to the question in the reply to my post, the engine does not smoke at alll from the tail pipes. would using a leak sealant solve the problem if it is a small leak around the water pump? the water pump was replaced prior to this problem happening.