If you have a locked signature for an sstate task where the sstate package isn't available locally (but is fetchable via SSTATE_MIRRORS), you'll get a warning when bitbake starts about the sstate package not being found. You can see this within the extensible SDK if you revert SIGGEN_LOCKEDSIGS_CHECK_LEVEL back to "warn" (you may need to do this from the build system end before building the ext SDK) and then try to do an update after adding something new to the image - see bug 8883 for the steps. There shouldn't be any warning here - we should probably be doing the check after running through the setscene tasks so we actually know if the sstate package is really available or not.
I think this might be indicative of an underlying issue - see bug 8883.
Reassigning back to Paul to either: a) confirm as a duplicate of 8883 b) confirm if the fixes in 8883 also fixed this c) reassign it back to me if there is a continued problem This might have also been the fetcher PREMIRRORS change which was reverted.
I'm fairly sure this got fixed - at least it does appear to be working as expected now.