"Podmore was arrested by Ministry of Magic watchwizard Eric Munch, who found him attempting to force his way through a top-security door at one o'clock in the morning."
-- from a story in the Daily Prophet, 7 Sept 1995 (OP14)
A security guard is called a watchwizard in the Wizarding world. Eric Munch was the watchwizard on duty in the Atrium of the Ministry of Magic when Harry visited for his hearing (OP7).
Variation on 'watchman,' typically used for a person who watches or guards a building at night or when it's empty. Munch was on duty during the busy day but also at night, judging by his apprehension of Podmore at 1 o'clock in the morning.