re: …how did you arrive at that conclusion?
In the Selection Info palette, expand the "Group of Graphics". The "TC Surface" object is on the "Measurements OH" layer, while the parent is on "Posts". Since this came from a Symbol Library, the post in the Library is set to the "Measurements OH" layer. I would recommend changing that Symbol to Layer_0, unless there is a need for it to be on the "Measurements OH" layer. With the symbol on "0" and the Group is exploded, it will acquire the groups' layer; i.e. Posts. If it has a layer already assigned, such as "Measurements OH", it will retain that layer name.
Layer_0 is always there for use by the program for a variety of things. It is usually best to leave it on.
The $Constraints layer is a BUG and can't be removed. I believe in a future version they may give the user the option to remove these dollar bill layers; $Constraints, $Construction, etc.
V17—V21, 2015, 2016, 2017
Designer, Deluxe, Expert, Basic, Platinum
Windows 10, 64-bit