"Sounds like a spell. It isn’t."
-- Hermione Granger (CC1.17)
A verbal abbreviation (slang) used by the UK police to refer to a “missing person”. A misper report is usually made directly to the police.
When Albus Potter and Scorpius Malfoy do not show up at Hogwarts for their fourth year the Minister for Magic, Hermione Granger, informs the Muggle Prime Minister, who will arrange for misper reports to be made to the Muggle police. Draco Malfoy is unhappy about "relying on Muggles to find our children" (CC1.17).
The British word "misper" is in the Oxford English Dictionary. It is a contraction of the two words "missing" and "person".
The UK Missing Persons Bureau is part of the National Crime Agency. Contrary to popular belief, you do not have to wait 24 hours before reporting someone missing.
From the Web
UK police information and guidelines on how to reporting a missing person: http://www.missingpersons.police.uk/en/resources/report-a-missing-person
About the play: http://www.harrypottertheplay.com