In The Husbands of River Song (the 2015 Christmas special), River Song says she has often stolen the Tardis, did her thing, and then returned the Tardis to where/when it was, without the Doctor ever knowing anything about it.
Is that really her only method of time traveling? Stealing someone else's time machine? Then how does she get to that town, at that time, in the first place? And if she has a means of time traveling of her own, why does she need to steal the Tardis?
I'm doubtless missing something obvious. What is it?