A procedure had been amended in the Editor. Because it was under version control, Toad prompted to save and "Continue with Check-in?". On clicking 'No' the procedure was still checked in.
Steps:
5. Say 'No'
Object gets checked back in when it should remain checked out.
Note, however, that no revision was created
WORKAROUND
No new revision has been created, and the changed object has not been compiled, so simply check-out again if you want to continue working with the object, or close the Editor tab without saving.
STATUS
Fixed in version 12.11. The latest release can be downloaded here.