This will be a long post to share some observations, not yet confirmed.
While in my first months of ownership, i was obsessing over efficiency and i found out that if i do city driving with lots and lots of traffic stops, my consumption was way higher than without stop/go traffic. This is quite common and expected, but while i was perfecting the slowest starts ever (single digit %s) performed and i annoyed hundreds of drivers behind me... still my efficiency was not all back. I also perfected stopping to red lights by using regen only, not one pedal mode, and annoyed even more drivers. Still not good.
Then i noticed that while i was stopped, the car would use between 3-6% power, but not moving an inch. I dismissed it as a UI glitch for weeks. And then one day while parking i noticed that the slow creep uses about the same power, i was making a 20 point parking by that time.
So, my theory was that if i stop really slow by only regen braking and maybe a feather bit more brake to fully stop, the power would go to 0% and when i lift the foot of higher on the brake it goes to around 4% (sometimes more or less, but not much). The car is 99% of the time not moving at all (flat streets where i live), but my theory is that the car is indeed using some small power but not enough to move the car. Too early to say, but for over a week i try to fully press the brake pedal then release and at least i never see that 'vampire drain' and it feels like my city consumption is somewhat lower.
Weirdly, i never saw this mentioned in any of the hypermiling videos.
Would love to hear if this is noticed by more people?
And to the OP, it seems like the car was not on a 0.0% incline street and that small power output was not enough to hold the car in place, so gravity did its thing sadly.
Edit: i realized i don't fully lift the foot off the brake...