It is not unusual for homeowners to go away on a summer vacation and return to find roaches in their home. Usually this is because the water in the traps beneath their kitchen and bathroom sinks has evaporated and allowed these insects to travel up through the sewer pipes into the home.
Prior to leaving for a vacation you should flush your toilets and turn your sink faucets on for 30 seconds or so to recharge the water in these traps. Ideally, this should be done every two weeks by your housesitter. If your home is only being checked monthly, have your housesitter add about a tablespoon of vegetable oil to the sink drains before running the water. The oil floats on top of the water and keeps the water inside the traps from drying out. This simple procedure should keep your home free of these pests while you are away.