Recently we left our 17 y.o. home alone, for the first time, while we went out of town (this was a big step for Mom!).
If/when we do that again, we will make sure that there is a gun available ~ just in case.
Given how well she shoots, how responsible she seems (at least around guns, boys may be a different matter entirely) , and given that it is YOUR HOUSE you are talking about, with what the police would surely call an arsenal, I'd be amazed and stupefied if there weren't.