2000 Sienna

I took our Sienna for the 90k schedule maintenance, which included replacing the water pump in addition to the other items and since then it looses coolant. The jobs was not done by the dealer, but all the parts were from the dealer as I purchased them myself. I took it back to the mechanic who did the job and he kept it for two days presure checking the coolant system and could not find any problems. Any experience with this issue? any suggestions would be greatly appreciated. Thanks.