1. Yes, you wired the NEMA 14-30 receptacle correctly. That receptacle has two hot connections, each 120V but on different phases, resulting in a 240V potential. So if you take a voltage tester you will find it reads 240V between the two rectangular slots on the outlet, and 120V between either rectangle and the neutral or ground. You can check this using your voltage tester, but it sounds like you did it correctly.

2. Figuring out where that wire went is going to be tricky. If you can't find it visually or by process of elimination, the best tool to help would be a tone & probe kit. You'd hook up the tone generator to your mystery wire (after double-checking that it's not live) and take the probe component with you. When the probe is near the toned wire, it will beep, even if it's a few inches away inside a wall.

I've labeled your image, to help you understand what's going on.

Off to the left, the grounding electrode conductor enters the box and terminates at the grounding bar. The feeder coming in the top of the disconnect has three wires, two ungrounded (hot) conductors, and a grounded (neutral) conductor. The two ungrounded (hot) conductors terminate at the disconnect, as they should. The grounded (neutral) conductor terminates at the grounding bar, as it should if this is where the service is grounded.

The feeder leaving the bottom of the panel has two ungrounded (hot) conductors, which terminate at the disconnect as expected. The grounding conductor terminates at the grounding bar, as it should. And the grounded (neutral) conductor terminates at the neutral bar. Unfortunately, since this appears to be where the service is grounded, the grounded (neutral) conductor from the lower feeder should be connected to the grounding bar.

As it's wired now, the grounded (neutral) from the lower feeder is connected to an isolated neutral bar. Which means that the grounded (neutral) conductor feeding the panel is floating, or not electrically connected to ground. Without a reference to ground, the voltage potential between either of the ungrounded (hot) conductors and the neutral can be anywhere between 0 - 240 volts.

Solution

Service grounding location

If the disconnect panel is where the service is grounded, you should move the white wire from the lower feeder to the grounding bar. Or you could bond the neutral bar to the grounding bar, using an appropriately sized conductor.

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Not service grounding location

If this is not where the service is grounded, you should move the bare conductor from the upper feeder to the neutral bar.

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Based on what you've said in comments; and because it's a 3 wire feeder and not a 4, it appears that this is where the service is grounded.