If you have ever tried to purge a textstyle, layer, linetype, etc. in a Civil 3D drawing, and you are prevented from doing so, but you cannot find the object in the drawing?
If the object cannot be found in the normal places (referenced by a drawing entity, dimstyle, mleaderstyle, etc.) – then there is a good chance that the object is being referenced by a Civil 3D style. But how do you find out which one? Head over to the Autodesk App Store and grab Style Component Find and Replace. This add-on will allow you to search for objects (Layers, Textstyle, Block, Linetype, and Plot Style) referenced in Civil 3D styles, and optionally replace them.
Note that for layers, you can also use LAYMRG to merge (and subsequently delete) the offending layer onto layer 0, but if you choose this route, be sure to run AUDIT afterwards. If not, the deleted layer will be missing from the Civil 3D styles and corrupt those styles. LAYMRG is not a Civil 3D command and it does not know to make the layer substitutions in the Civil 3D styles.
