|This needs a stretch.|
Needed: Physical appearance.
The transmitter was a homing beacon used on Scooby-Doo's person to track his location during a dognapping case.
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Scooby-Doo, Where Are You!
Scooby-Doo played decoy as a champion show dog to lure out the local dognapper and find the other stolen dogs. Fred Jones tucked the transmitter into his fancy-looking new dog collar to keep a tab on his location when the dognapper eventually caught him and took Scooby back to his hideout. The transmitter was connected to an antenna mounted on top of the Mystery Machine which was in turned connected to a monitor which showed a technical read-out of where he was.
- The surveillance equipment is too sophisticated to be brought from a store, and even if it could the gang wouldn't likely be able to afford it. Of course there's Daphne Blake's vast wealth (which hadn't been established yet), but she didn't seem to use it at any time for a case no matter how necessary it may have been. Velma Dinkley is supposed to be capable of building sometimes highly advanced inventions, but that wasn't established at this point and was even strongly suggested in Spooky Space Kook, but retconning it may be a possible solution.