House wiring question

[DJJ]

I have a receptacle in my house that's wired as shown in the drawing. I've looked at others, and they're all the way you'd expect them: black wire to black-wire terminal, white wire to white-wire terminal, and bare to ground, whether there's one or two sets of wire coming into the box. According to my breaker panel labels (and the fact that everything else still seems to work with the breaker shut off), this receptacke is the only thing this circuit feeds, but it has wire in and wire out as though it were in the middle of a run. I want to run a wire from this receptacle to provide power out to a shed. Both sets of wire are your ordinary 12/2 house wire, but with yellow sheath. Any idea what's going on here?

[280plus]

It looks like it is being fed from the right and the left is to a switch (?) that energizes/deenergizes the top receptacle only. Meanwhile the bottom receptacle is hot constantly. Check with a meter to confirm that power is fed from the right in your pic. Is there a wall switch in the room that you never knew what it was for? If not is it possible a switch was eliminated during a remodel and the wire is tied together and hidden in the wall?

Semi educated guess is the best I can do.

[K Frame]

Yep, looks like a switch loop.

[DJJ]

Doh! That's exactly what it is. The top outlet powers the garbage disposal, which is switched up above the kitchen counter (why didn't that occur to me earlier?). Clearly (now), the white wire on the left takes power from the right black, up the left branch to the switch, and the black on the left brings it back down to the upper outlet. The tab connecting the hot terminals has indeed been broken off. So I tap into the lower terminal.

I swear, it's a miracle I've never forgotten to unzip my fly before taking a pee.

Thanks.

[cfabe]

Yep. That white wire should be wrapped with black electrical tape or marked black with a marker to show that it's really a hot.

[DJJ]

Update: I looked in the box with the switch, and the white isn't marked with black, so I marked it. Thanks for the heads-up.