My business used to be next door to a company that sold supplies to exterminators and as a result I got to know a few of them. It was confided to me that rodent contracts are one of the more lucrative contracts one can get, especially with governement contracts. I am outside NYC and one guy had part of a contract for the city and as you say all he had to do was go replace the bait in the stations periodically. Sealing up the house is probably the single most important thing you did! Once they get in, then they cant get out and no more can get in!