13

Here's an example from a shower stall at work.

metal disc on wall

I've seen them in other types of walls, including exterior ones.

25

Plumbing pipe cleanout cover. You can remove the silver coverplate and you’ll have access to the sanitary drain pipe.

  • 1
    You also see plastic ones for getting at the electric wiring, often with junctions behind them. – RedSonja Nov 28 '18 at 13:56

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