You list the locations where you will be at any given time and they will go wherever you are–you can also fill out a change of plan form if you are going on a short trip or something of that nature. If you are somewhere else, they can go meet you. Brian Clay one time was on his way to pick-up his kid from school, and they just followed him there and then back.
Basically, they will go wherever you list on the form within 24hrs before you are declared as having missed a test, which is why it is suspicious to say the least how somebody can miss more than one test in a short time frame. Even if they miss it, they have a month to give a written reason as to why they missed the particular test. Depending on what the IAAF rules, the test may not be declared as missed.
They will not look for you for 24 hours. The time frame is more like less than 4 hours and often times less than 4 hours. You must be where you said you will be at all times.
This requirement is what led to the british 400m runner getting into trouble. On the day of her second missed test she was not at the track where she normally trained. She had gone across town to another track because there was an event being held at her regular venue. The day of her third missed test she was at an event speaking to school children set up by her federation, but had not changed her location with the drug testers. In all three cases of her missed tests she was available within 4 hours, but they did not wait that long.