I have no idea...but I wonder if it belongs in the center armrest...kinda looks like the shape of where the phone would go.
But better yet...turn it over and look for a BMW 11 digit part nbr...if you find it...just go to any of the online part databases (like
BMW Parts Catalog) and paste the 11 digit (or the shortened last 7 digits) into the part nbr search feature.
EDIT: LOL!...just did a quick search using a
2012 X5 35d as the search vehicle in the database...again, I don't know if this is the part or not...but the shapes are VERY similar. Again, check for a BMW part nbr...then look it up in the part database...and you'll have your answer.