Possibly-bogus error about missing dkms.conf file during autoremove

Updating the kernel, or anything which requires rebuilding initramfs (including "apt-get autoremove") produces error messages like this:

    run-parts: executing /etc/kernel/postinst.d/dkms 4.2.0-19-generic /boot/vmlinuz-4.2.0-19-generic
    Error! Could not locate dkms.conf file.
    File: does not exist.

But dkms seems to be working fine -- virtualbox modules get rebuilt as they should be. There might be some other package which is broken with respect to dkms.conf, but the message gives no hint about where to look.

I tried to figure out where the message is coming from, but got no hits (other than log files) from
  # find /boot /etc /lib* /bin /sbin /usr /var -type f -print0 | xargs -0 zgrep -l 'Error.*Could not locate'

I will attach /var/log/apt/term.log

ACTUAL RESULTS:
   Error saying dkms.conf is missing

EXPECTED RESULTS:
   No error, or else an error which indicates which module is broken or the path of the missing file

(If the underlying problem is not in the above postinst script, but in another module which produces the un-useful iagnostic, please redirect this bug to that package).

$ locate dkms.conf
/etc/modprobe.d/dkms.conf # this one is empty
/home/jima/flashcache/flashcache-wt/src/dkms.conf
/home/jima/flashcache/src/dkms.conf
/usr/share/virtualbox/src/vboxhost/dkms.conf
/usr/src/bbswitch-0.7/dkms.conf
/usr/src/nvidia-352-updates-352.63/dkms.conf
/var/lib/dkms/bbswitch/0.7/build/dkms.conf
/var/lib/dkms/nvidia-352-updates/352.63/build/dkms.conf
/var/lib/dkms/vboxhost/5.0.12/build/dkms.conf
/var/lib/dkms/vboxhost/5.0.9/build/dkms.conf
/var/lib/dkms_SAVE/vboxhost/4.3.10/build/dkms.conf
/var/lib/dpkg/info/dkms.conffiles # all files listed herein exist

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: linux-image-extra-4.2.0-19-generic 4.2.0-19.23
ProcVersionSignature: Ubuntu 4.2.0-22.27-generic 4.2.6
Uname: Linux 4.2.0-22-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: jima 2484 F.... pulseaudio
 /dev/snd/controlC0: jima 2484 F.... pulseaudio
CurrentDesktop: Unity
Date: Mon Dec 21 14:33:19 2015
HibernationDevice: RESUME=UUID=692a674b-d118-4d8a-906c-ecd9d1accff7
InstallationDate: Installed on 2013-08-06 (867 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
IwConfig:
 eth0 no wireless extensions.

 lo no wireless extensions.
MachineType: System manufacturer System Product Name
ProcFB:

ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.2.0-22-generic root=UUID=cb77f6cf-d3da-493f-bad6-2c7bdf5f1550 ro quiet splash
RelatedPackageVersions:
 linux-restricted-modules-4.2.0-22-generic N/A
 linux-backports-modules-4.2.0-22-generic N/A
 linux-firmware 1.149.3
RfKill:

SourcePackage: linux
UpgradeStatus: Upgraded to wily on 2015-11-09 (42 days ago)
dmi.bios.date: 09/21/2010
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 1408
dmi.board.asset.tag: To Be Filled By O.E.M.
dmi.board.name: P6T
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: Asset-1234567890
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr1408:bd09/21/2010:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnP6T:rvrRev1.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer