Police regularly go to usps centers and look for "suspicious packages" looking for all types of drugs. They get these packages separated and run the addresses and corresponding names through a database called "accurint" and if the names and addresses don't jive and if it is from a source state(just about any state), they may do a dog sniff. Based upon these suspicious factors such as name not matching address on accurint and source state info, they may seek a warrant to open and possibly plant a transmitter from a fed judge and deliver. Obviously they are looking for the bigger packages like a shoe box that weighs 20lbs lol..but you get my point.