Thanks for the additional info. ...Though an expired certificate means they haven’t been tested in a while to confirm they haven’t picked up some bad/confusing habit, or that they can still detect the specified materials (dogs senses of smell can be burned out by some of the compounds they are used to detect).
I would think the legal principle would be that behavior of a dog without a current certification cannot be used to establish “probable cause” for a search.
Who knows how it’s actually interpreted these days.