New
#11
Using the Sharing tab's Share button causes the folder's permissions entries to no longer be inherited from the parent. This causes the lock icon to show up when sharing is cancelled, because inheritance is not restored.
To fix, after unsharing:
Right click ->
Properties ->
Security ->
Advanced ->
Change Permissions ->
Check "Include inheritable permissions..." ->
Apply ->
then remove all the non-inherited entries
This restores the folder's permissions to the way they were before sharing occurred, and the lock icon is no longer shown.
If instead one has shared using the Sharing tab's Advanced Sharing, the inheritance is not broken, and all is well.