How does a 3-way motion sensor switch interact with the mechanical switch on the circuit?
To make a true 3-way motion detector switch -- as opposed to a motion detector with a remote, or a motion detector in series with a normal switch -- the motion detector has to be able to monitor whether power is being drawn through either of the travellers and which traveller it is currently connected to.

If no power is now flowing, to turn on flip to the other traveller.
If power is flowing, to turn on stay unchanged.

If no power is flowing, to turn off stay unchanged.
If no power is flowing, to turn on flip to the other traveller.

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