I wonder if they were quite audible? Maybe if they were that is how the cast noticed.
CATS,
Yes it did happen at the palace. Yes there was also that story of the cast during all shook up. But, the theatre is totally coincidental. I don't find these stories to be fabricated at all for a couple of reasons. First off they aren't posted on here every other day. If they were to some frequency I would have trouble believing them. But, considering the fact that they are not posted constently gives them some sense of credibility.
Also, it is known that people who have no sense of theatre manners at all do these sorts of things. I can't tell you the number of actors blogs I have read where they talk about this. Lea Salonga had an entry in her blog during her run in Les Miserables about how there was a young couple making out and it was visible to the whole cast.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll