OK, cool. I'm going to close it up and wait for your advice.Yes that is a two-way switch arrangement for at least one of the light fittings with another switch somewhere else also controlling the power to the light fitting, where the available cables have been used, breaking the normal colour code rules because of what is available. Another example of pathetic registered electricians at work, too damn lazy to ensure the correct cables with the correct colour codes was used in the original construction.
I will have to think about how you can use the SONOFF arrangement with a two-way switch setup before suggesting how the wiring should be. In the meantime, that lone black wire will have to proven to NOT be a neutral wire before you do anything. And you will have to determine which two-way switch is the "master" ie. the one where incoming supply is connected.
I have a diagram somewhere of what a two-way switch arrangement can look like. Unfortunately, there are a few variations also and I am pretty sure at least one of those variations will not work too well with a SONOFF switch added.
i would go with disconnecting the wires so that only the one is connected to the light, make it the master and then just wire the other switch up to have power and make a scene.If that "stray neutral" is indeed the wiring for an old-style two-wire double switch arrangement where whoever installed it didn't bother to use another color code, then I don't think you will be able to duplicate the exact behavior of the two existing switches. Usually they're wired such that if the light is on, you can switch it off at ether switch, and if its off, you can also switch it on at either switch. So far as I am aware, you could wire two Sonoffs such that you could switch on at either switch, but they would both have to be off for the light to be off.
If you decide to change to having only having one switch per bank of lights, then you'd have to identify which the red is that is paired with that black, and disconnect both the red and black at both ends, and disable. Then wiring the Sonoff would be basically the same as the previous switch.