I am in the process of tracing all home wiring. I am down to one switch box. The box was covered with a plate. Inside the box are 4 black wires, three white wires, one red wire, and ground. One of the blacks is a pigtail capped. The red is capped. The blacks are all tied together. The whites are all tied together. It seems like this was at one time a switched outlet that they disconnected. but I don't know. All this comes from one two wire and one three wire. Any advice for how to figure out what outlet was switched and how to wire it?
Switch the breaker off. Install a switch and attach the switch terminals to the black pigtail and the red wire. That should reactivate the switch except.
You will need to learn what "tabs" are.
And then you will need to open up receptacle(s) around the room and look at how they are wired. Look for receptacles that have a red wire attached to the receptacle (not just passing through) and whether it has a black wire also. If it has both, look to see if the tab is broken off.
It happens all the time where novices replace receptacles (e.g. right before a house sale), and don't know anything about tabs and fail to break off a tab that needs to be broken off. This renders the switch inoperable. I could see a frustrated novice then removing the inoperable switch so they didn't need to explain to the buyer why the switch doesn't work.
To make the switch work again, you'll need to find ALL the "shoulda been broken off" tabs. Even one left on will break the switch.
By definition, a switch that needs its tab broken off will have wires on both screws (or the backstabs right next to the screws).... I suspect they will be different wire colors.
Don't just break the tab on every receptacle with both screws populated. Most of the time, the receptacle is simply being used as a splice block and power is meant to go through the tab.