Clutch engagement point moving at random, doesn't seem to be a hydraulic problem

I've had little hints of this problem on my AE92 before, which caused me to start monitoring clutch fluid levels carefully, but levels are stable. Sometimes when crawling in traffic, the clutch "bite point" will move from its usual position to being very close to the floor. Then I release the clutch and it's back to normal. I'm thinking my clutch fork or TOB could be sticking somewhere. I do hear a slight creak when moving the clutch pedal, although it's been like that forever, even when I had the 4AF with a slightly different transaxle. I've tried putting the car in 1st and holding the clutch down for an extended period of time, the car never pulls forward and it doesn't reproduce the problem. Clutch is an Exedy Stage 1 (single kevlar full disc, sprung) with a few years of autocross, track days and occasional street driving on it.

Engagement moving seems like it has to have a hydraulic component to me, even if it's something else causing the hydraulics to misbehave.

Missing/broken clutch pedal return spring causing the master not to get the refill port open to the reservoir consistently?

I suppose something dragging in the TOB/fork could stop it returning fully as well, but spring seems easier to check first