Can a live (capped) knob & tube hot wire be left disconnected, without the neutral (it's removed) in a new switch box now fed by a modern 20amp circuit and romex? The reason it is still live is because the circuit operates other lights in the house.
Knob and tube is no longer listed and approved, it should be de-energized and abandoned when encountered within the scope of any new work after being replaced with listed cable assemblies suitable for the location (in your case, non-metallic bonded cable, aka Romex).
That said, it sounds like what you have in the box used to be a switch leg where the original knob and tube feed part of the leg is no longer present (hence, the absence of the neutral), and through the process of removing it the lighting circuit was changed so that the leg ended up being always hot. What you should do is just disconnect it from whatever fixture is now back-feeding it. Be gentle, that stuff is brittle and the insulation can just disintegrate in your hands as you handle it.
If that's the case, then you probably don't have a multi-feed (two circuits on the same phase sharing the same neutral) going on, it was just a switch leg, but you should probably consider retiring the rest of the old wiring anyway.
If you can't find where it's being fed, your immediate concern is now to ensure that the box is suitably grounded if it's metallic, you can test this by attaching one end of a voltage tester to any known hot leg, and the other to the box itself. You should read around 120V and be able to confirm the presence of some kind of physical ground connected to the box. If that's not the case, call an electrician and have them straighten it out.
Still, the best guidance in these cases is just get rid of it where / when you see it, especially if it's been disturbed recently. That old feed is probably not going to cause you any problems, but please consider erring on the side of caution and having that old lighting circuit re-fed.