This is a hard one. Could be she was sending out a help signal and you picked it up. I hate to suggest this because it puts you in an awkward position. You might be able to help, you might not. Maybe it's just a coincidence, but I don't believe in coincidence. What I would do is focus on her picture and meditate on it for awhile. Go into a somewhat of a dream state and project myself to where she is. Now the things you see in the spirit world are not always the same as what we would see with our eyes. If you could pick up on something that gives you an indication of where she is wouldn't that be really cool? I think you should try it even if nothing comes of it. Don't feel bad if nothing happens. If your stressed out about it that could get in the way.
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.
There is also something called lucid dreaming where you are asleep but yet have more conscious control of what you do and where you go in dreamland. This is often activated during naps or when you wake up during REM sleep and then go back to bed. Some people lucid dream and go visit friends and relatives in their dreams. And I've ever heard of friends lucid dreaming together and meeting up to share an adventure. But as opposed to regular sleep, in lucid dreaming you mostly control where you go, who you see, and talk to, whereas regular dreaming you are not really in control. Astral travel by comparison is where you through deep meditation will visit other worlds. It's almost like landing on another planet. It's an experience that you don't really control you just experience it. A lot of people talk about noting a silver cord or tether attached to them when they astral travel (the link between our world and the outside world). So based on your description, I don't think you are experiencing astral travel.
Remember as you get deeper in understanding spirituality you will learn that nothing is a coincidence. People are placed in our path for a reason. As are challenges. And since we are here in this life to learn many lessons, we are always being directed by our spirit guides to learn and advance to our given challenges. Your day dreaming could be lessons. Although I suspect that you are starting to see visions of real events that have either happened or will happen. Since you have recently started relaxing and working on your gift, you are noticing that it is strengthening. That's confirmation for you that this is the real deal. Keep working on those abilities.
I did want to say that for me, my visions only work when I am relaxed and not trying or thinking of using them. It's almost as if someone drops an idea or a little movie inside my head and it autoplays. It sounds like that it may be the same for you. You may want to practice meditation to truly relax your mind and see where it goes. It may result in more visions or you may actually experience astral travel first hand. Just make sure you do some reading in this website's library about astral travel as you will only want to do that with proper protections.