What exactly is the procedure for dealing with an alarm set off on a
tube train?

This morning, at approx 10.15am, I boarded a Circle Line train
eastbound at Kings Cross. As the doors closed, someone pulled the alarm
(not sure where). The doors then opened and the (female) driver said
'We are aware that the alarm has been pulled. Would the person that did
so please make themselves known to a member of staff'.

This strikes me as rather odd. If it was someone taken ill, how would
this work? If it was someone being attacked, why let them run off the
train?

Jonathan