If the documents were checked out to the profile you were logged in under, you would see green check marks instead of the padlock, so you must be logged in under a different profile. Do you know what type of authentication is used in your installation (Windows or standard)? If you are using Windows authentication and use multiple computers, you would have a profile on each computer which would show 'checked out to' information in the format "UserID\ComputerName" (or maybe it's UserID.ComputerName). It is possible this is your problem... you checked them out on Computer A, but now are using Computer B.
Otherwise, you can look in Tools - Profiles and see what profiles exist on your computer. Perhaps you have two profiles for yourself that use very similar, but different UserIDs (this would apply to Standard authentication).
If you can't figure it out and you have administrative priveledges, you could force a cancel check-out (any changes since they were checked out would be lost!!!) via the Administration menu. Once this is done, you should be able to check them out.
One of my UPK 11.1 clients were experiencing the same problems. They would check out topics, work on them on one day, come into the office the next day, log onto UPK using the same profile, and then would not be able to check in the documents.
I personally think that this is a "whoopsy" between server and client, but how do you reproduce it consistently?
To resolve we did the following:
1. Force check - in with admin account.
2. Check out data/content and make changes as required using appropriate profile
3. Check in data before exiting/closing UPK developer (this ensures that the profile that checked out the content checks it in again)
4. Login in using same or different profile, check out content, work, check in content, exit.
Basically you need to check in content each time to ensure changes are committed to the library. 99% of the time they are able to leave files checked out for a number of days, come back, continue working on them, and then check them in again - hence why I say it is erratic and a bit of a "whoopsy", because some days it gets "iffy" and then for the next couple of weeks you can use UPK as you did before without checking out and in all the time.