When I export to CSV from the search page and select the Group(s) field it includes past groups as well as current groups
Example of steps to reproduce:
1. Create new individual First Name=Bill, Last Name=Smith, tick Group Major Donors
2. Edit groups for this contact - remove from group Major Donors and add to group Newsletter Subscribers
2. Search for "Smith, Bill" and to bring up just that record in the search results
3. Select Export, choose fields First Name, Last Name, Group (s)
4. The Group(s) column in the exported CSV field has Newsletter Subscribers and Major Donors in the field even though the contact is not longer a member of Major Donors group.
Reproduce on the demo site today (CiviCRM 9263) and on own installation (CiviCRM 1.7.beta.9284)