Sorry, I've been at work on some other projects. There's a lot of AI research in tracking which might be useful here, but it would require some adaptation. I'm thinking of something like a particle filter. It would track of a collection of possible locations for the player, weighted by likelihood. The weights are updated based on sensing, by increasing weights with positive evidence (sightings, noises) and negative evidence (seeing empty space where the player might have been). It should also be possible to do things like combining the knowledge of different guards and adding evidence from things like open doors or extinguished lamps. Of course, the problem here is not to find the player's location (which we already know) but to simulate the guards' lack of knowledge in a realistic way. I think a scheme like this could give the guards an apparently "rational" way of working out where the player may be. The question then remains of how the guards should use this knowledge, but that will require more thought.