I recently put up shelving using wall studs as support. I used an
electronic stud finder, then of course made sure the screws went into
the stud and not the wall. However, I just found out about the magnet
test and it indeed is helping me find an additional stud a lot easier
than the stud finder did. However...

For the 12 x 36" shelf I already installed, the magnet test is not
finding a stud where I had installed one of the shelving brackets, but
about 3-4" to the right of the bracket, yet I know both screws for that
bracket went into a stud. There are no wires or outlets near this
section of the wall. Can studs sometimes be only 3-4" apart for some
reason with one stud having no nails driven into it for the drywall
installation? How can I tell for sure? Could it be anything else
holding that shelf bracket?

Thanks!