That does look as if it could be soot. The wires themselves don't look damaged (melted, charred).
So there may have been some kind of appliance or device plugged into an outlet there, which failed and caught on fire, which presumably also damaged the receptacle. Or the receptacle itself may have failed and started smoking. In either case, the receptacle was never replaced, possibly just because it wasn't convenient or the previous owner never got around to it, or didn't know how.
There wasn't necessarily an overcurrent condition. But if there was, it could have been due to all kinds of transient conditions.
However, there's also a chance that the overcurrent was due to something like a floating neutral in your panel, which can be really dangerous. As in appliances spontaneously catching on fire or blowing up, and so on.
Do you have any electrical problems anywhere else in the house?
The wires in this box are connected straight through, which indicates that you probably have healthy receptacles further "downstream," in which case there's no reason why this box should be a problem with a new receptacle in it.
For peace of mind, you may want to hire an electrician to check things out all the way back to your service panel, to make sure that all of the connections are tight and you don't have any intermittent failures (like a cracked wire that carries enough current 90% of the time, but then overheats and moves causing voltage fluctuations).
If you disconnect the wirenuts, only one of the black wires should be live, and it should only show current when connected to one of the white (grounded/neutral) wires, and it should also show current when connected to one of the ground wires. Those would be the three wires that lead back "upstream" to your service panel. The other wires carry current on to additional outlets. In both cases (grounded and grounding, or neutral and ground) the current should be a steady 120 volts give or take a few volts, with or without a load on the circuit.
It's possible the panel you have illustrated is one of several such panels serving power into the unit, so you may need to do an exhaustive search for any others.
It complicates things if this building has more than one unit. Wiring to the other units could be still energized obviously... And this could create false readings on test equipment (capacitive coupling) -- or there could be actual cross-connection between the units, so your unit might not be fully de-energized unless others are too.
Cross-connection could be a load in your unit served out of their circuit... Or even a neutral cross, where their load is served from their panel, but it uses the neutral on your panel, and if you sever that neutral wire which has current flowing through it, the upstream side will be energized to 230V.
A lot of testers are complete junk, or are not made for house wiring and need some skill to successfully apply.
A common night light is surprisingly useful. It is simple, honest and not particularly reliable, which means you must check and double-check it. I honestly find them more useful than 3-lamp testers, though those are OK for a final check for new outlets.
Plain old analog voltmeters with the moving needle are useful.
Digital voltmeters (DVM) are useful but a little tricky. They are so sensitive they can pick up micro-currents from capacitive coupling, which happens when a dead wire runs physically parallel to a still-energized wire. So you have a DVM a reading of 109 volts on a 120V circuit that you thought was turned off - plug a night-light in, and the voltage mysteriously disappears.
If the electricity is powerful enough to illuminate any light, even an indicator light -- that's not capacitive coupling, that's genuine power.
Best Answer
OK, First a "chirper" measures the magnetic force around an ac current carrying conductor, but it does not identify the strength or the amount of power of that conductor. So you may be picking up trace voltages. Since you mention knob and tube also romex, I am assuming that it is an extremely old system and it has been modified probably more than once. Knob and tube and some older romex systems do not have grounding systems. It also means you may have a switched neutral which was legal many years ago (like 75 years).
I suspect that you are getting a certain amount of feedback power through the neutral from circuit 8 and circuit 18. When you add the comment that ceiling lights are constantly burning out it may be confirming that you are experiencing "floating" power and the variations of the power is not good for the life of any lights or equipment. For example, if your lights are dimming in the kitchen when you turn on a hair dryer in another room, that would also indicate a "floating neutral". The only other reason you would be burning out lights is that you're buying cheap lights. If you are not experienced in electrical power I would suggest professional help. If you want to try and labor through it USE EXTREME CAUTION.
The first thing they teach in electrical apprenticeship training is to always treat everything as if it were hot unless you are absolutely sure it is dead. In your case turn off the entire panel.