Water pump leaking after replacement

I replaced the water pump and everything went well. I tested it by allowing the car to run for twenty to thirty minutes and I did not see any leaks. The customer called and said that it is leaking again. I have not checked to see if it is coming from there or somewhere else. My question is, when I removed the water pump there was not any sealant used on the old surface so when I put the new water pump on I did not use any sealant. I am wondering should I have used some sealant when I put the old one on? Also, I am working outside and it is a little cold out. I did not change the water pump housing I only replaced the impeller side of the water pump and the belt.