Police were called to a Britain's Got Talent audition after a bugging device was found under the judges' table.
The listening equipment was discovered before Simon Cowell, Amanda Holden and Piers Morgan began a second day of filming for the ITV show.
Suspicions were aroused when it caused interference with recording equipment at Manchester's Palace Theatre.
Cowell, the show's creator, said it may have been planted by a freelance journalist hoping to gain an exclusive.
He said: "This shows the extent to which people will go to get inside knowledge on what is going on."
A Greater Manchester Police statement said officers were called to the theatre in Oxford Road on Thursday at 1230 GMT.
The show invites talent of any kind to perform, and the first series, won by Opera-singing phone salesman Paul Potts, attracted fire-eaters, acrobats, dancers and singers.
Ant and Dec will host the programme when it is broadcast later this year.
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