I'm with those who say ask the hostess. Explain your irrational fear. Yeah it might be a tough conversation to initiate, but if the hostess was that casual about the rat, then I imagine she's unlikely to get too bent out of shape over you mentioning it. The conversation about why you never go back or never eat at her house again will be just as tough to have.
Another explanation of why a rat got in and was wandering around while there was a lot of activity is if a cat brought it in. Our best hunters have brought mice, birds, rats, chipmunks, large bugs (I hate bugs!) in to play with them and nibble on them in privacy. Sometimes they get away....I suspect the cats sometimes let them go so they can experience the chase all over again.
My husband and I have had to chase many a small critter out of the house while restraining a cat.