Looks like a type of basalt to me. Some types of basalt will signal (sweetly) on a detector. Even though they seem to have a weak or no magnetism various types of basalt can have a high iron content - this is why the outer layer of an otherwise blackish rock is brown/rusty.
The iron content in different types of basalt & basalt in different areas can vary widely - this is why some signals & some doesn't; some areas are prone to high iron content basalt all over; some areas have a few that signal but most don't & other areas don't give much if any trouble at all.