Had a similar issue. Place is 100+ years old and I wanted to hang a table in the kitchen, but that's not the sort of thing you do without a stud. Luckily I had 2 things going for me.

1. Tiny neodymium magnets (strong enough to hold themselves on the wall)
2. Access to both sides of the wall.

I used the cubes to mark every metal object found. Then I repeated this on the other side of the wall to corroborate. I discovered the 9ft stretch of wall had no standard stud spacing. Rather a single 4in wide stud (maybe two 2x4s?) in the middle of the wall.