That isn't an estimate of P(UAP). The following two propositions are very different:
The first of these can be true without the second -- maybe there are aliens but they never come here. (That was rather the point of the Fermi paradox.)
The second can be true without the first -- maybe people have actually seen angels, or time travellers from earth's future, or spontaneous rips in the fabric of spacetime.
It would be a powerful tool to be able to dismiss fringe phenomena, prior to empirical investigation, on firm epistemological ground.
Thus I have elaborated on the possibility of doing so using Bayes, and this is my result:
Using Bayes to dismiss fringe phenomena
What do you think of it?