Welcome aboard Mika! I'm fairly new here myself. Thanks for posting all of this. I think the warnings are pretty much covered. I'm glad you did scold them in this case. They are either actually children or being children and you are in the position of a parent or guardian here, so I'm glad you did and that it worked! I have no idea what the truth is behind them, but just to throw out another idea. The objects disappearing and appearing and messages seem like poltergeist activity and one explanation for that is that the person at the centre of the activity is subconsciously causing it through an unrcognised ability. Following that line of thought then Andrea and Susan would be aspects of your own subconscious mind, much like a childhood toy or imaginary friend and this is you talking to you (no shame in that) with odd physical effects happening. You mentioned that Matt could be mean sometimes and while you wouldn't hide a credit card to freak him out, perhaps you might subconsciously want it to happen so... And then when you realise that it's to do with you you think of Andrea and Susan and so (because they are good kids) it returns? That wouldn't make Andrea and Susan not real as the mind is a strange and wonderful place, but it's another explanation. It sounds like you enjoy having them around.