I’ve measured with a multi-meter, and the 5v line is totally cold when powered off. It has a small residual reading from the capacitors as they discharge, but nothing that could power a sensor for any length of time.
Yet, exactly like in your setup, if I connect a Motion module to a Dock with a powered off ring it will carry on ticking as if nothing was different. Bonkers!
I read around 2.6v across the 5v rail of the plugged in sensor, so yeah… it can only be powering itself parasitically from the open-drain i2c lines with, presumably, the onboard decoupling capacitors - and perhaps even the caps on the dock itself - offering enough storage capacity to smooth out any wrinkles in the power supply caused by actual i2c comms (which, granted, are intermittent anyway).
It does highlight one clear flaw in my Dock firmware, though, and that’s that the module scanning code still runs even when the Dock has cut the power to the power ring. This is an oversight, and I should check to see if power is on before attempting to communicate with any modules.
I have a few things which need to be rolled into a Flotilla Dock firmware update, so I’ll add a fix for this to my todo list and it’ll be in the next version.