1) I clamped around an elbow that started taking me around the building; had to open it up and found a grounding wire!
2) & 3) Had a fiberglass pipe that was in a storm drain network at an old industrial facility. Never did figure out what it was.
4) Opened up a handhold labeled "Fiber Optic", thought geez that's some interesting looking fiber and when I clamped around the cable my clamp was buzzing loudly! Found out it was primary power....
5) Did a locate at a daycare and thought this was too good not to share!