I'm trying to wall mount a tv. Finding the best suitable place, I made sure to assess the integrity of the stud behind the drywall by tapping (percussing). I noticed a hollow portion of vertical stud as seen below. I drilled a hole just to confirm it and indeed there is a structurally unusual gap approximately 17 inches. Behind the wall is a bathroom with the plumbing side, but it seems too high to route the plumbing.
So what's with the gap? Is it safe to mount a 30lbs tv on top of the gap on the stud?