Oh boy let's see where to begin.
Pokemon Crystal- Mysteriously disappeared, I noticed it was gone once and I never found it. I am not even sure when it went missing, but it was only a few years back I think, but it could be related to the next two.
Pokemon Pearl- Lost it uh on one Easter I believe last year or the year before. However I am pretty certain my cousin stole it, why? Because he stole my Pokemon cards when I was 12 year old. Only difference is when I was 12 my mum, his mum and brother gave a damn and told him to give it back. Now I am 23 (22 at the time) and nobody really cares and my mother, or my sister don't even believe me when I say he stole it. And his mother knows he probably stole it too, she told me he tends to steal stuff.
Pokemon Emerald- This was after Pearl and Mario Kart (Yes he stole my Mario Kart DS as well at another time). I know it was him, because I know only three people were in my room that day (My birthday I think). His youngest brother and his younger sister, and him. (4 and 12) Now I know for a fact the youngest couldn't have reached Emerald and was only in there for the morning, his sister was only in there for the first half and then I was playing with her for the rest of the day (And I have let her borrowed DS games before, she knows all she needs to do is to ask, unlike her brother, hell I told her she could have some of the change that lies around my desk when she pretended to take them)..
Another thing is this, Pokemon Emerald was placed between Pokemon LG and another game. If it was just knocked down or lost then the game on top of it would have been knocked down too, but to steal it he placed the top game back (I even took a picture of my room before he came, and the picture clearly shows where Emerald was and where it wasn't now).
My aunt, his mother says he's probably sold them by now though >.> So I guess technically they aren't lost, they were stolen. However to this very day Pokemon Crystal still remains a mystery, is it lost somewhere in my room or did my cousin also steal that game without me noticing it. (He's really dumb though, same day Emerald was taken I left a fifty dollar note clearly out in the open, as to try and catch him. He took Emerald then the 50 though)