It appears that .siginfo files are not being accessed from local or NFS-mounted SSTATE_MIRROR (file://), so their timestamp never gets updated. If you periodically clean SSTATE_MIRROR based on access time, they get deleted, even though they are still being used, leading to errors like:

WARNING: quilt-native-0.64-r0 do_populate_sysroot_setscene: Failed to fetch URL file://Ubuntu-16.04/52/sstate:quilt-native:x86_64-linux:0.64:r0:x86_64:3:5227556845cbc224f05726d1633de4e5_populate_sysroot.tgz.siginfo, attempting MIRRORS if available 
ERROR: quilt-native-0.64-r0 do_populate_sysroot_setscene: Fetcher failure: Unable to find file file://Ubuntu-16.04/52/sstate:quilt-native:x86_64-linux:0.64:r0:x86_64:3:5227556845cbc224f05726d1633de4e5_populate_sysroot.tgz.siginfo anywhere. The paths that were searched were: 

In this case, corresponding file://Ubuntu-16.04/52/sstate:quilt-native:x86_64-linux:0.64:r0:x86_64:3:5227556845cbc224f05726d1633de4e5_populate_sysroot.tgz is still there, but not its .siginfo

Everything works fine for HTTP-based SSTATE_MIRROR, because .tgz and their corresponding .siginfo files are being downloaded, updating both files timestamps on the server.

So, regardless of SSTATE_MIRROR access protocol, when .tgz is being used, corresponding .siginfo file should probably be "touch"-ed to update it's timestamp to prevent this issue.