I used a 3/32 drill bit for a pilot hole into my dry wall, directly in between 2 wooden studs that were about 32 inches apart, and about 18 inches from the ceiling (of a 1 story house). I mistakenly thought that there was another stud there because the magnetic stud finder was picking something up there. I immediately hit metal behind the drywall, and when I ran my magnetic stud finder along this line, I realized that almost the entire length of this vertical line was metal. What did I hit, and do I need to open the drywall to see if I caused any damage to anything important?
If the magnetic stud finder is showing response across a good part of the stud cavity then there is probably a metal duct in the wall. This could be a stove hood vent, a dryer vent or a heating duct.
If the magnetic finder is showing just a narrow range near where you drilled the hole then the item in the wall might be a metal pipe such as a sewer vent stack, an iron water pipe or an iron gas pipe. Since the first two examples would be pretty rare to use iron pipe I would put biggest odds on a gas pipe.
There is another possibility that you ran into a metal electrical conduit.
One possibility not mentioned, there may be wires behind the plate if they were not able to locate them far enough back the plate prevents the wires from being damaged by nails, screws ect,,, the plates are usually galvanized steel if that is the case.
Bought a new house and was hanging TV's and pictures. I use a stud finder and then measure. Found what I thought was another stud and drilled with a new 3/32 cobalt drill bit for a pilot hole. Hit something hard and like a idiot put more pressure on it (STOP)...went thru a 2" gas line. Square on...could not have done a better job with a drill press looking at the thing. Silver lining was after the plumber came and did the repairs the 10 psi pressure test showed 4 small leaks elsewhere and he also found badly kinked SS flex line to stove. So $$$, lesson learned, and the the fire dept knows where my house is.