In a large house that is often empty, a running toilet can go unheard for a very long time.
The bathroom nobody uses
With five or six bathrooms and a house occupied a few weeks a year, a toilet refilling itself every few minutes may run for months. It is worth walking the house and simply listening at each bathroom door before you leave, or asking whoever checks on the property to do the same.
Seals dry out when unused
Toilets that sit unflushed for weeks can develop small leaks as seals dry and shrink, and the trap water evaporates so odour comes back. Flushing every fixture during a caretaker visit prevents both.
If the toilet rocks
Any movement means the seal underneath has failed and water is going into the floor. On upper storeys that means the ceiling below, and in an empty house it means weeks of it.
Heavy use in short bursts
When the house is full it is genuinely full. Flappers, fill valves and handles wear at that pace, and the off season is the sensible time to deal with them.