If a section/topic is restricted to a grouping and that grouping gets deleted, the topic still appears to be restricted to the grouping.
To replicate:
Create a grouping called temp_group.
Edit the last topic. In the Restrict access section, select the grouping you just created in the 'Grouping access' drop down box
Leave all other settings as default. Save changes
Topic should now display 'Restricted (completely hidden, no message): Available only to grouping temp_group.'
Delete the Grouping you just created
The last topic still says 'Restricted (completely hidden, no message): Available only to grouping temp_group.'
If you Delete the last topic and then add a new topic it will still display 'Restricted (completely hidden, no message): Available only to grouping temp_group.'
If you edit the topic, the grouping is no longer in visible in the'Grouping access' drop down box.

The work around...
create a grouping called 'temp'
create the same no. of topics that were there previously
edit each topic and assign temp grouping to it, save it, and then unassign the group
delete the sections just created
delete the grouping.