You could always just be honest and say, “hey _____, I heard through the grapevine that you were retiring? Is this true? I’m just trying to sort out fact from fiction / would prefer to speak directly with you than deal in gossip.” Keep it simple and without pretext.

Seems personal. But then, I need to know...

well, that's part of your in. For starters, if she's announced it to some people, you should be among the first to know since it would affect you. (Frankly, if your pastor is gone and he's as of yet unaware, it was rather unprofessional for her to start telling people...) You need to know for professional reasons, not just to be in-the-know for curiosity's sake.

Why do you need to know?

You want to know, because you have aspirations. But unless you have collaboration between the sites planned for after January, you don't have a need.

IMHO, there is nothing you need to ask your colleague about. She's leaving. It's not up to her to choose what happens after that, it's up to the pastor. He may prefer to continue having two part time musicians, because it gives cover etc. He may want you to step up. He may not. You need to talk to him. How to do it depends on your relationship with him - you more about that than we do.

Argentarius,

You could approach the other musician or the pastor with the same context: People have told you, but you haven't heard it from the horse's mouth (so to speak) so you'd like to know how to react when people tell you this next time. That way, you make yourself part of a team (which you may be, anyway) trying to stop the spread of gossip and/or misinformation.