switches don't need and rarely have a neutral core, ie, just a live in & switch line back which is often incorrectly coloured black or blue, leading people to think there is a N.
There is then normally a 3core with earth between switches to convert to a 2way circuit.
Occasionally people have tried to change a 1 way cable to 2 way by using the earth core sleeved over as a switch line, but wrong on 2 fronts, first no earth in circuit & insulation not continuous where sleeving meets cable sheath.
To add to what sparx has said from your description it sounds like the 2 way light has picked up a feed from hall switch, is using a twin for strappers between switches and is using the red core in a twin with the black chopped out as the switch wire to the landing light fitting. Thats why the black is not being used - I guess!
Nothing wrong with that as long as it works and all switches have an earth.
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