A Metal Detector is a device used to track any trace of metal, usually found underground. It is first seen in the episode "Walking Small."
Metal detectors change in appearance throughout the series. In "Walking Small," metal detectors are silver in color and have headphones. In "Penny Foolish," a metal detector is green in color.
In "Walking Small," SpongeBob SquarePants is seen using a metal detector at Goo Lagoon. Sandals wants to use SpongeBob's metal detector as his broke and SpongeBob allows him to use it. However, Plankton, who was trying to teach SpongeBob to be assertive at the time, attempts to get SpongeBob to take back his metal detector but this only helps Sandals find buried treasure.
Another metal detector is seen in the episode "Selling Out," where it is used by Mr. Krabs presumably trying to search for money on the beach. However, Mr. Krabs gets rid of his metal detector after seeing a "Help Wanted" sign on the new Krabby O'Monday's restaurant.
In "Penny Foolish," a metal detector is used once again by Mr. Krabs who is searching SpongeBob's house for a penny. SpongeBob catches Mr. Krabs in the act and even wonders why Mr. Krabs isn't wearing his "metal detector handling gloves."