I do have some experience with what you are talking about and can share some info.
I would not say you are astral projecting. You don't usually do that in your sleep. You astral project by being in an awake but deep meditative state. It's more of a planned quiet time journey.
But, I do get visions while awake that are worth noting. They are similar to day dreaming except that day dreaming usually starts from your mind's own idea. Whereas a vision is usually a thought dropped in to your mind that was not something your asked for or were focusing on. And right before visions my mind gets completely calm and clear of personal thought and then while my eyes are open I will stop seeing the present world and will instead watch a short movie in my mind like you referenced. Quite a few of my visions have foretold future events. While others I have been unable to connect to anything and just remember them to be lessons that I do not yet know how to interpret or use.
Dreams are a whole other ball game. Some dreams are completely your mind's own doing. And then spirits can also impact your dream if you are susceptible to that as an empath. And then last but not least, your spirit guides can send your messages and lessons through your dreams. I do know that a lot of mediums are given an abundance of info. through their dreams.
I would say that there is a pretty good chance your gifts are directing that missing girl to you. And if she is unfortunately now deceased then either her spirit is reaching to you for help or for some reason your guides are wanting you to get involved. A lot of mediums help their communities by getting involved with the police and families of missing persons due to their ability to channel in to the event that caused a death or disappearance. It's very possible that this is your skill set.
If any of this scares you you can try to block it and deny your gift. I have a friend of mine who is a very talented psychic who does not want any part of it and has blocked most of her abilities. But if you want to develop your gift I would suggest believing and being receptive to it. And definitely keep a vision and dream journal and try to connect the dots on the info. that you are receiving.