Some interesting comments in this thread... some helpful, and some making some big assumptions.
First of all, I'm certainly willing to try and improve my behavior for the benefit of us all (after all, we are all in this together, right?). As such, I believe I agree with the sentiment of what I think I'll term: "Charge early, Charge often" when using public charging infrastructure. Not only does this allow me additional margin for error in case of the unexpected, but frees a limited resource up for others more quickly.
In this case I didn't think a scheduled charge that was going to shift my charge session back by a couple of hours to be a big deal, especially considering it was already something like 1am and there was no contention at the time. As I said earlier, I'll also print some placards up, but I'll be frank: I'm not inclined to accept phone calls at 4 am to go move my car.
Now to clear up some presumptions several folks have made:
1) My car WAS charging. When I checked my charge status in the morning, significant mileage had been added. It had simply been unplugged in the middle of the session about 60 minutes before it was complete.
2) My car has no indication charging is still in progress. With my firmware version, my charge ring, dash, and center console all go dark several minutes after the car is locked. Those who assumed you could tell by observing my car that a charge is complete are incorrect.
3) It was not a chargepoint station with any sort of LCD readout of charge current, etc... It is a Tesla branded (model TS-70) charger. It DOES have a charge indicator lamp, but it seems to kick on as soon as the car is plugged in and handshake completed. This could perhaps be due to initial current draw for the battery cooling etc... similar to my HPWC. I would assume the indicator goes dark when charge is completed but this shouldn't have been the case for me (see #1 above).
4) This is a hotel with valet parking. Not only do I not park my car, I observed the valet's park Teslas and Volts in the spaces set aside for using charger of their own accord. Even when I didn't ask to come down and connect the charger via my J1772 adapter, they did this. They retained the key fob (not me) for purposed of parking, retrieving, or moving cars.
5) There are 4 spots that the single charger can service. I was parked in the rear-most one, which does NOT block access to the other three (and actually is the spot that would in turn be blocked if additional cars came).
Perhaps that provides some context for those that made some assumptions.
Incidentally, after an hour I came back down to unplug, and was going to plug the other model S in, but it had left.
(And finally to the one poster that asked for the plate of the other car, please... I'm not going to post car plates on a public forum, and I'm not going to call any user names out. If they are reading this then you've already made your point to them. )