We have had the same sort of thing with twins. The SKY box used to be underneath the Television on the stand. One morning, leaving the kids playing as you do, you no longer hear the television on and its all very quiet.
Walk in the room, TV is on with the SKY box saying "Please Insert Card" or words to that effect.
Now at this stage the kids cant talk or tell you where they have put it (Just about 1 at this point). We have no exterior Aerial for the house and Internal ones dont pick up a good signal where we live.
All downstairs was thrown about looking for this card for what must of been almost 2 hours. Called SKY and arranged for another card which will take up to a week to come through. No tele for a week OH NO. Decide's to go out to the pub for lunch and a drink. Puts feet into shoes and voila Kids had put the card right at the bottom of my shoe so noone could see it.
This along with destroying most of my boxset DVD's that were in the lounge (In excess of 30 Boxsets built over many years) I was close to putting them up for adoption.....