The more complicated you make things the simpler it is for them to break, ie someone walks by the bathroom while someone is in the shower.
But your on the right track, ie the use of Bayesian Probability. Take lots of input and determine the probability of something being true. While this is used generally to determine whether someone is ‘at home’, the same theory applies to whether someone is in the shower. Its pretty commonly used in HASS setups, and the same theory applies.
