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.
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.