Metal detectors change in appearance throughout the series.
The metal detector is silver and has headphones.
"Selling Out" and "Penny Foolish"
The metal detector is green.
Role in series
SpongeBob uses a metal detector at Goo Lagoon. Sandals wants to use SpongeBob's metal detector, as his broke, so SpongeBob allows him to use it. However, Plankton, who is 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.
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."