2000 Dodge Caravan water pump/leaks

Had a garage mechanic replace water pump. Went on 400 mile trip. Before returning after another week of short drives, I noticed it had lost some fluid. Added about half gallon. In the 4 weeks since it has continued to leak, but only when the engine is cold. Isolated leak to the timing belt cover area which covers the pump. Mechanic worked on it again, but only to re-tighten and seal the outside bolts (so he said). That changed nothing. Assuming he did in fact change the pump (I know it's kind of a nuisance to re-check because of having to remove the timing belt and tensioners) what is the most likely cause? He's mentioned that he possibly didn't get the O ring properly seated. Could he easily have checked this or the weep hole with his second look?