DIAGNOSIS The machine is likely entering Auto+ mode and, due to low pollutant readings, reducing speed to a near-off state (Fan Speed 1), which is very quiet.
FIX Check the current mode. On the display or in the app, ensure it is not set to 'Auto'. If it is, switch it to a manual fan speed (2-10). In Auto mode, if the sensors detect clean air for a prolonged period, the fan slows to its minimum setting, which can be inaudible.
IF THAT DIDN'T WORK The Night Mode schedule may be active. In the Dyson Link app, go to the schedule tab and check for any programmed 'Night Mode' periods. Night Mode sets the fan to a very low, quiet speed (Speed 4 or lower) and dims the display, which can be mistaken for being off.
HEADS UP There is no automatic shut-off feature outside of a programmed timer. The display will always show a standby light unless fully unplugged. If the unit is truly powering down completely, there is a potential internal fault.