It's the not informing you that I see as the red flag. I'd just ask them to keep you apprised of any "practices" they might be having, and what their plans or goals are. HC to AC is a really short communication chain, and there shouldn't be any unknowns happening. You never know if it's just someone taking initiative and making that extra effort without thinking, or if they're recruiting for their own purposes, or just trying to push one girl ahead of another on the roster. Also that jealousy monster could creep into your team chemistry if it's not being handled fairly and out in the open.
I'm also curious.. Why one girl? Is she the best player or is she a girl that could use extra work on her game? Is this something to get individual practice time for each girl so it's maybe another girl next week. Or is this something that's going to build resentment among the players that don't
Unfortunately, the grooming thing is real and a real concern for any organization. No one on one time with any coach and girl that's not their daughter. Keep multiple adults around at all times if you can. I've had a lot of training with a lot of different organizations on what to look for, and that's a biggie. One accusation or even suspicion, founded or unfounded and the damage to both sides is unrecoverable.