S&R is started when you don't arrive.

Well, here's the detail I'm looking for: How do they know you don't arrive -
they have to be told that you will. How do they know when you will arrive -
they have to be told the time to expect you. So, "some time" goes by and you
haven't arrived. S&R is started.

How much time is "some time"? (I'm not asking for an exact number - I'm well
aware that it may not exist).

And the "action" in this case is being told that you will arrive, and "some
time" going by without an arrival. Yes?

Jose

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