Finding hard-to-find studs
Option 1: Measure 15 1/4 inches minus the thickness of the drywall (or panell etc.) from another exterior wall. If that misses, measure 23 1/4 inches minus the drywall. If that misses, go to the other side and measure from the opposite wall.

Option 2: locate an outlet. A stud will (probably) be on one side or the other.

Option 3: read more on How do I locate studs in my wall?