Clean out? The hole in the cap was from the screw.

Is the square part 1.25x 1.25"? The cap on my clean out is.

It does look like a plumbing cleanout, but its hard to see clearly. I would try to unscrew the cap to verify that it is a standard plumbing cleanout, and then put on a new cap.

You can buy standard cleanout covers at any HD or Lowes. If you recut that to a square hole, or make a little bit larger round hole, many of the covers just pop in and are held by spring retainers behind the drywall. You can also frame it with wood and mount a hinged cover, but its probably not worth the trouble in a garage.

Bruce

Covered because it is ugly as-is. Not unsafe to touch, possibly sewage if it is a drain line instead of just a vent line just wear some disposable gloves. Would have been better off painting it white instead of drilling a hole in it. Get a new plug, clean up the area, and paint it.

It's just a PVC cap, I'm not a plumber but I think they're just access points if you need to clear a clog or inspect the drain line or something. I'd consider replacing it with a new plug if PO drilled a hole in it to hold that bowl on.

Odd spot to have it, I'd either paint it white so it matches the wall, replace with a new white piece, or cover it up with a cabinet/etc.