Rodents are quiet, nocturnal, and good at staying out of sight. By the time most homeowners see a rat or mouse, the infestation has usually been active for weeks or months. The good news: rodents leave evidence everywhere they go. Here are the ten signs we look for on every inspection — and what each one tells us.
1. Scratching, scurrying, or gnawing sounds at night
Rodents are most active between dusk and dawn. Light, fast scratching overhead usually means mice; heavier thumping or dragging sounds often mean rats. Sounds in walls or ceilings — especially along the same “routes” night after night — are a classic first sign.
2. Droppings
Mouse droppings are about the size of a grain of rice with pointed ends; rat droppings are larger (½–¾ inch) with blunt ends. Fresh droppings are dark and soft; old ones are gray and crumbly. The location matters: along walls, in corners, near water heaters, in the backs of cabinets, and along attic runways are hot spots. Don’t dry-sweep them — rodent droppings can carry hantavirus and other pathogens. Mist with disinfectant before cleanup.
3. Gnaw marks
Rodents’ teeth grow continuously, so they gnaw constantly — on wood framing, plastic pipes, stored boxes, and electrical wiring. Fresh gnaw marks are light-colored and rough; older marks darken over time. Chewed wiring isn’t just a repair bill: it’s a genuine fire hazard.
4. Nesting material
Shredded insulation, paper, fabric, and dried plant material gathered into loose balls — tucked into attic corners, inside wall voids, behind stored items, or under a vapor barrier in the sub-level. In attics, look for insulation that’s been tunneled through or gathered into beds.
5. Grease and rub marks
Rats have poor eyesight and travel the same routes repeatedly, leaving dark, greasy smears from their fur along walls, beams, pipes, and entry holes. A well-worn “run” with rub marks tells us the infestation is established, not new.
6. A musky, ammonia-like odor
An active infestation produces a distinctive stale, musky smell from urine — strongest in enclosed spaces like attics, sub-levels, and closets. If a room smells “off” and you can’t find a source, rodents belong on the suspect list.
7. Unusual pet behavior
Dogs and cats often detect rodents long before people do. A pet that fixates on a wall, stares under appliances, paws at a particular corner, or is suddenly agitated at night may be hearing or smelling activity you can’t.
8. Damaged or flattened insulation
Insulation that’s matted down, tunneled, discolored, or stained is a reliable sign of rodent traffic — and it’s also a performance problem. Contaminated, compressed insulation loses R-value, and urine-saturated material keeps attracting new rodents with its scent.
9. Chewed HVAC ducts (and mysteriously rising energy bills)
Flexible ductwork in attics and sub-levels is a favorite rodent target — they chew holes for shelter and nesting material. The result: conditioned air leaking into the attic, dust and allergens pulled into the system, and energy bills that creep up without an obvious explanation. If your house smells musty when the heat kicks on, the ducts deserve a look.
10. Daytime sightings
Seeing one mouse doesn’t necessarily mean a big problem. But seeing rodents during the day — especially rats — usually means the population is large enough that competition is pushing them out at unusual hours. That’s a sign to act quickly.
Found one sign? Assume there are more
Rodents rarely leave just one clue, and a single rat can become a colony faster than most people expect — a breeding pair can produce dozens of offspring in a year. The right response is a methodical inspection of the whole property: roofline, foundation, attic, and sub-level, with every entry point documented. That’s exactly what our $350 inspection covers — a full evaluation, an illustrated report you keep, and first-line protection on the spot. If you hire us for the work, that fee is credited and matched, giving you $700 toward the project. Call (707) 978-6024 (Napa/Sonoma) or (415) 740-5170 (Marin/S.F.) to schedule.