Great question — answering as the UphillTech team:
If it is acceptable that the warning light goes off if the motion sensor has switched off (i.e. did not detect motion for long enough), then you could wire your SPDT (aka 3-way) switch between the motion sensor and the mechanical timer. Motion sensor to the common terminal of the switch. Position 1 to the mechanical timer and alarm system Position 2 to the "reminder" light. By putting the switch of the reminder light before the timer, the reminder light will also turn on if the switch is set to "alarm disabled" at times that the timer also keeps the alarm off. If it is a hard requirement that the "reminder" light is on continuously while the alarm is disabled, you will need the relay and...
Full answer by Bart van Ingen Schenau: https://diy.stackexchange.com/a/332048 (CC BY-SA 4.0).
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