onlooker
n
person who watches sth happening (without taking part); spectator 旁观者
By the time the ambulance had arrived, a crowd of onlookers had gathered. 救护车赶到时, 围观的人已经很多了.
Then, her composure regained, she was ready to set off with the porter's assistance to search for nay intruders who might still be lurking in her flat.