One unexplained activation is not a fault yet, but it is information, and the information is perishable. The panel knows which device operated, the building knows what was happening at the time, and both of those are easier to establish in the next hour than in a week. A single alarm properly investigated is usually the cheapest fault you will ever fix.
Establish what actually operated
Before resetting, read the panel: which zone, which device, and what time. On an addressable panel the event log holds it anyway; on a conventional one it is gone once you reset. Note whether it was a detector or a call point, because those are two completely different investigations and confusing them wastes everybody's afternoon.
The causes that leave no trace
Steam that has dispersed. An aerosol sprayed in a corridor. An insect that has left or died invisibly in the chamber. A momentary electrical transient. A cleaner's dust that has settled. All of these are gone by the time anybody looks, which is why the record of what was happening at that time is more useful than the inspection afterwards.
Vaping and similar
Vapour is a dense aerosol and will trigger an optical smoke detector at close range, in a lift lobby, a stairwell, a hotel room or a toilet. It usually shows up as repeated activations at one device in a place with no obvious fire risk. This is a siting and detector type question, and sometimes a building management one, but it is a real and identifiable cause rather than a mystery.
When to stop treating it as a one off
Two activations at the same device, or three anywhere in a short period, is a pattern. At that point stop resetting and start diagnosing: the same device operating repeatedly is telling you something specific about that location. Feed the log book entries to your maintenance contractor rather than asking them to find an intermittent fault with no history.
Questions people ask about fire alarm went off for no reason
Should I reset the panel straight away?
Read and note the zone or device address first, and confirm there is no fire. On a conventional panel that information is lost on reset, and it is the only lead you have.
Can a spider set off a fire alarm?
Yes. Small insects inside the sensing chamber scatter light in the same way smoke does, and they are one of the more common causes of an activation that leaves no visible evidence.
Do I have to record a false alarm?
Yes. The log book should record every activation and what caused it. The pattern across entries is what allows a recurring problem to be diagnosed rather than repeatedly reset.