after the captain was made aware of the note and before the diversion, I do not understand the decision-making process.
Either the note is taken seriously, then land at the nearest suitable airport. Why wait for a fire alarm? Or wait for people on the ground, whose lives are not at stake, to make the decision? It might be too late then.
Or the note is not taken seriously, then just go on to your destination.
What is the logic here? "Oh, we have been flying for a while since we saw that note and there is no fire, now let's divert."
I don't believe this is covered in the SOPs
Decision making when your aircraft is running at near perfection is not made on the spur of the moment.
Information must be collected and weighed against what ifs.
Folks who don't have to make this kind of decision never seem to grasp this.