Professor Dweeb used his Ghost Detector to trace Slimer to the Metropolitan Hospital. Instead of catching Slimer, the Ghost Detector grabbed the chair he sat in. Mrs. Stone didn't take too kindly to the property damage and ejected Dweeb.
The Ghost Detector is a vaguely geometric box with a tiny, spinning radar antennae on top. When a ghost is detected, it unfolds into a much larger, trap-like device with saw-like teeth.