A toilet quietly refilling itself is the most common thing we find here, and on a large house with several bathrooms it is easy to miss entirely.
Guest bathrooms are the usual offenders
In a home with four or five bathrooms, the one nobody uses is where a running toilet goes unnoticed for months. It is worth putting your head round the door of the spare bathrooms occasionally and just listening.
Hard water wears the parts
The mineral content here stiffens flappers and fill valves so they stop sealing. Inexpensive part, quick visit, and it is the single most common cause of a water bill that has crept up without explanation.
Builder-grade fittings past warranty
Homes built out at the same time were fitted with the same components throughout, so they need attention around the same time. If yours is original and has needed parts more than once, replacing is often better value than a third repair.
Upstairs bathrooms
A failed seal on an upper floor drains into the ceiling below, which turns a small repair into drywall, paint and sometimes cabinetry. Any movement in the toilet is worth attending to quickly.