Yep, I think 'when in doubt bow' is just the right spirit. And, perhaps, when really in doubt, sit seiza and bow, if you're not already sitting.
What's the proper etiquette for say.. you are at a seminar somewhere with your sensei from your dojo, and the class is being taught by someone other than your sensei. Is it still correct to refer to your sensei as "sensei" during that class, or should there be one and only one "sensei" during the class i.e the one teaching....
Two thoughts - 1. ask your teacher what he or she thinks is correct and do that. 2. If don't have the chance to ask, using 'sensei' too much is better than not enough. Again, when in doubt, bow.