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Describe what you could see and hear separately
A note saying “the fireplace stayed on” can mean a visible burner flame, air from a grille, a light effect, or sound at the vent. Record the position of the observer and what continued. If a burner appears to stay lit despite the normal off command, follow the appliance's instructions for that situation and get professional help. Do not treat a continuing flame as an ordinary fan delay.
Read the fan section for your control
On equipped appliances, Hearth & Home Technologies' IFT-RC400 user guide describes delayed fan startup and continued fan operation after the flame is turned off under the conditions stated in that guide. Those timings belong to the named system. They do not establish a universal delay or prove the behavior of a different appliance. The same guide distinguishes the fan feature from a power vent.
Use the observation you already have
If the household noticed the sound fade after normal shutdown, note approximately how long it lasted and whether air was felt from a room grille in normal use. Also note whether the routine has changed from prior visits. Do not open the cabinet, touch hot surfaces, unplug components or repeatedly cycle the burner to diagnose the sound.
Give the returning owner an unambiguous note
“Fan continued after the burner went out” tells more than “would not turn off.” Add the model, the written explanation you found and any unresolved warning message. If the sequence differs from the owner's manual or there is an unexpected noise, leave the appliance off until the provider can advise. A gas smell calls for leaving the area and contacting the supplier or emergency services from outside.