What are the signs of mice infestation in winter?

Droppings are usually the earliest and most reliable sign, because mice produce them constantly wherever they travel and feed -- NC State Extension notes a single mouse can leave 50 to 75 droppings in a single day. Fresh droppings are dark and glossy; older ones dry out to gray and crumbly, which is a useful, low-effort way to tell current activity from an old problem you've already dealt with.

Gnaw marks and nighttime scratching tend to follow close behind. Mouse incisors grow continuously, so gnawing isn't optional behavior for the animal -- it's a constant, low-level activity, and fresh, light-colored marks in cardboard or wire insulation mean the mouse was recently at that spot. Scratching concentrated after dark is consistent with mouse behavior specifically, since house mice are nocturnal.

None of these signs are unique to mice on their own, which is exactly why the honest checklist names the look-alike for each one rather than treating every dropping-shaped speck as proof. Coffee grounds, potting soil, and cockroach droppings all get mistaken for mouse droppings regularly. House settling and HVAC ductwork produce sounds people describe, almost word for word, the same way they'd describe scratching in a wall.

The practical fix is to look for more than one sign in the same area before concluding you have an active problem -- droppings plus a gnaw mark plus a rub mark in the same corner is a much stronger case than any single sign alone.

This is the part that's easy to skip and shouldn't be. Signs tell you mice are active in a general area -- they cannot tell you the nest location, the size of the population, or whether wiring inside a wall cavity has already been damaged. Nobody has X-ray vision into a stud bay or under a slab, and any claim to the contrary should be treated skeptically.

What that limit means practically: use the signs above to decide whether to act (set traps, seal obvious gaps) and where to focus, but don't expect them to tell you "how many" or "how bad" with any precision. That's a fundamentally different question, and it's the one a professional inspection is built to answer.