I have a run that has an individual light switch in another room and at the door to go upstairs there is a switch that turns off "ALL" the lights (I'm calling it a "master" switch.)

There are 2 rooms that work only from the "master" I want to beak off one more room that isn't used so often with it's own switch to keep the lights off unless I go in there.

I expected to just run a branch off the HOT(black/red) side of the run to a new switch in that room from the box near the light fixture.

Before I started I wanted to be sure that I was getting everything right. This is the first time I have done wiring work in this new(to me) place. So I pulled the plate at the "master" switch and the only wires in the box are "WHITE"!

I suspected that it was just the wire on hand and no one put tape on the ends to note that they were really from the HOT "colored" side. So, I traced the white wire back. It goes straight to the circuit box neutral bus bar. Everything appears to be in hard conduit and has a professionally done feel.(neat compound bends)

I've never heard of this before.
Is this OK?
Can I still wire as I normally do?
Any special considerations?

Many thanks from a kind of handy guy...

Switching the neutral is not allowed. You should have an electrician come in and evaluate your system looking for other possible Code violations, and determine what is needed to get your wiring system back to normal.