My experience with roaches and plumbing tells me this.
Roaches love to use sewers as a source of comfy atmosphere, water and food. They also make a great highway to travel around town unseen.
With that being said, they usually do not leave the pipes unless there is a hole in a pipe somewhere or a dry trap. Some people think that they will go through the effort of swimming through the trap seal especially if they see one swimming around in a toilet or sink drain.
They typically become trapped in a fixture after falling in from the edge and can't get out, like the spider in the bathtub scenario.
Most exterminators will inject/spray a powdered pyrethrin insecticide in drains and vents when the situation warrants, but the focus should be on food storage areas, cabinets, cluttered basements and the other obvious dark undisturbed habitats. Look into the possibility of crated food or furniture or anything that has recently been brought into the house that could have transported a colony inside. Storage warehouses for retail products can be a roach heaven and provide a free ride to a new environment.
And as always clean, clean and clean....every nook and cranny . While cleaning look for areas of congress where piles of their little black dotted feces will be noticed. There's a reason why roaches gather and stay in some areas and the reason can be discovered when you find lots of roach poop there.
A reputable and professional exterminator is usually worth the cost if you follow their advice before it turns into an infestation.
After treatment, the home owner should look around for cracks and crevices that should be sealed as well.
