It is not either of those items, it looks like a 1x1 flat tile cut in half, not the 1x1, 24246, half circle that looks like it was extended or elongated 1x1 square flat tile. It came with one of the following sets, lunar lander, Easter egg, Yoda or rough terrain crane. I have look through the parts list at the end of the assembly instructions but I have not noticed it thus far.
It certainly sounds like Tile 1 x 1 Half Circle (24246/35399), which @One_Click_Off posted above... I'm unaware of any other tile that is similar. If it's not Tile 1 x 1 Half Circle (24246/35399), I echo @HowardE's suggestion to post a pic, I'm super curious now! :-)
Comments
Sounds like these could be one of two “tiles”
Tile 1 x 1 Quarter Circle (25269/48410)
Tile 1 x 1 Half Circle (24246/35399)