fly on the wall
An unseen observer or listener, as in I wish I could be a fly on the wall when she tells him she's pregnant. [Mid-1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for fly on the wall
Since 1984, Newsweek has made a cottage industry out of its fly-on-the-wall post-election coverage of presidential campaigns.Newsweek's Greatest Hits
The Daily Beast
May 5, 2010
He could play the fly-on-the-wall in Louis the Goon's life as long as he wanted.