Without seeing any fan specs, impossible to say.
Could be wrong label on the fan, different fan, or the fan may have an internal pwm regulator so the Op is seeing just the average current.
What you described is rubbish.
The motor would never look like starting with a relay coil in series with it.
What does work is as I have done many times , wind a few turns of enamelled copper wire, placed in series with the motor, sufficient for the motor current, (16awg) and place a reed...
Thought you might show where it comes from or what piece of equipment.
Just because the Anode is connected to ground doesn't necessarily make it a zener.
Prime example is in a bridge rectifier.
Op should not be getting a collector to emitter reading for 1941 as in #29 nor should there be an open on the base to collector in both dirtections.
Haven't looked any further as Op apparently is getting confused as to just what is being measured where.
I'll leave it to you to sort out but in my...
Time of flight sensor positioned at the top of the container should detect the movement.
One of those "suck it and see" situations.
Sensors and clone Arduino relatively cheap.