Missing Refrigerator water line valve
Search inside the kitchen cupboards, especially under the sink.  If that doesn't turn up anything, some idiot probably drywalled over it when they did some "improvements" before flipping the house.  

In that case, you may have to drill some small holes in the walls to find it.  You can get pretty cheap (like under $30) USB inspection cameras you can hook up to a laptop, so you only have to make small holes to look around.

Once you find it, you can cut the drywall away and make it accessible.  Google "access panel" for ready made doors to mount over it.  If you want to be really nice to future you, put a label behind the fridge describing where the valve is.