You will need to explore heavy activity areas or where there are stains, holes, eves, attic/crawl space, insulation etc. Borescope is a wonderful tool. Or doing it the old fashion way and cut test patches. This nesting was found behind a fixed cabinet panel, ended up removing just under 200 mice after a good seal up and trap session using snap traps.On the other side of the cabinet divider were "clean" dishes. Based off of the infest rodent jobs I've done, you'll most likely need to remove old insul and replace it along with toilet areas that the walls/ceilings have absorbed urine/droppings. Set perimeter and interior traps and set dozens of them, along with a good seal up and you should knock them down. As far as having a "professional" come in. One of my last rodent jobs I've done the homeowner had a very well known company come in to seal up the house for 3,800, before I started my trap program with them. They installed hardware cloth, bent trenched in metal, new metal along fascia boards and mortar for the foundation. Looked amazing but the mice found a way in through the corner board and in return allowed them to bypass the metal and enter through the soffit into the attic/living space. Infestations aren't as hard to handle as many think...that is..if heavy equipment isn't needed.