The simplest explanation for the phenomenon is that the open collector transistor is not a perfect resistance. It's a semiconductor and is still connected to other sources of voltage. Pico amps or Femto amps of current can still flow if there is a current path to ground. In fact, every point in a circuit is always connected to every other point in a circuit through some combination of resistances, albeit extremely high ones. Even capacitors leak current through their dielectric in tiny amounts, thus possessing a measurable resistance.
A good multimeter with input resistances in hundreds (or thousands or tens of thousands) of Mega-ohms will allow such a flow to ground but will not load down the voltage that appears on its input, so it gets read. An old, low impedance input meter will load down the tiny current and will read zero volts.
What you end up reading with a modern meter is the voltage across a voltage divider, (consisting of the transistor and the meter input) of hundreds (maybe thousands) of Mega-ohms. You're practically reading ghost voltages.
Pico amp and Femto amp circuits can drive one nuts until all of this is clearly understood. The rules are the same, but nothing else is. When the exact capacitance of traces is a big deal and the dielectric value of your circuit bd material becomes a big deal too, you know you've entered another world of electronics.
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As an aside, my Fluke lab meter has such a high input impedance (>10,000 Mega ohms,) that the voltage taken from a reading won't even go away until I take another reading, except through the slowest of circuit leakage. I have to be careful when using it because if I take one reading, then take a different reading at a point that is actually an open circuit I will still read a value very close to the first one taken, instead of a new one.
I usually have a 10 Meg resistor across the meter input to avoid that occurrence, but must remove it for accurate high impedance readings.
As the input impedance of even cheapie meters has risen drastically over the last decade or two, folks sometimes run into such problems figuring out just exactly what it is they're reading. The best advice in that regard is to always remember that your meter is a part of the circuit you're measuring.
