root@X:~# grep update /etc/apt/sources.list* -R
/etc/apt/sources.list:deb [arch=amd64] http://repos.XXXX.net/debian-security/ jessie/updates main non-free
/etc/apt/sources.list:deb [arch=amd64] http://repos.XXXX.net/debian/ jessie-updates main non-free

root@X:~# dpkg -l | grep linux-image
ii  linux-image-3.16.0-4-amd64      3.16.51-2                          amd64        Linux 3.16 for 64-bit PCs
ii  linux-image-amd64               3.16+63                            amd64        Linux for 64-bit PCs (meta-package)

root@X:~# apt-get update && apt list --upgradable | grep linux-image
Ign http://repos.XXX.net jessie InRelease
Atteint http://repos.XXX.net jessie/updates InRelease
Atteint http://repos.XXX.net jessie-updates InRelease
Atteint http://repos.XXX.net jessie-backports InRelease
Atteint http://repos.XXX.net jessie-prod InRelease
Atteint http://repos.XXX.net jessie Release.gpg
Atteint http://repos.XXX.net jessie Release
Atteint http://repos.XXX.net jessie/updates/main Translation-en
Atteint http://repos.XXX.net jessie/updates/non-free Translation-en
Atteint http://repos.XXX.net jessie/updates/main amd64 Packages
Atteint http://repos.XXX.net jessie/updates/non-free amd64 Packages
Ign http://repos.XXX.net jessie-updates/main amd64 Packages/DiffIndex
Ign http://repos.XXX.net jessie-updates/non-free amd64 Packages/DiffIndex
Ign http://repos.XXX.net jessie-updates/main Translation-en/DiffIndex
Ign http://repos.XXX.net jessie-updates/non-free Translation-en/DiffIndex
Atteint http://repos.XXX.net jessie-updates/main Translation-en
Atteint http://repos.XXX.net jessie-updates/non-free Translation-en
Atteint http://repos.XXX.net jessie-updates/main amd64 Packages
Atteint http://repos.XXX.net jessie-updates/non-free amd64 Packages
Ign http://repos.XXX.net jessie-backports/main amd64 Packages/DiffIndex
Ign http://repos.XXX.net jessie-backports/non-free amd64 Packages/DiffIndex
Ign http://repos.XXX.net jessie-backports/main Translation-en/DiffIndex
Ign http://repos.XXX.net jessie-backports/non-free Translation-en/DiffIndex
Atteint http://repos.XXX.net jessie-backports/main Translation-en
Atteint http://repos.XXX.net jessie-backports/non-free Translation-en
Atteint http://repos.XXX.net jessie-backports/main amd64 Packages
Atteint http://repos.XXX.net jessie-backports/non-free amd64 Packages
Atteint http://repos.XXX.net jessie-prod/main amd64 Packages
Atteint http://repos.XXX.net jessie/main amd64 Packages
Atteint http://repos.XXX.net jessie/non-free amd64 Packages
Atteint http://repos.XXX.net jessie/main Translation-fr
Atteint http://repos.XXX.net jessie/main Translation-en
Atteint http://repos.XXX.net jessie/non-free Translation-en
Ign http://repos.XXX.net jessie-prod/main Translation-fr_FR
Ign http://repos.XXX.net jessie-prod/main Translation-fr
Ign http://repos.XXX.net jessie-prod/main Translation-en
Atteint http://apt.postgresql.org jessie-pgdg InRelease
Atteint http://apt.postgresql.org jessie-pgdg/main amd64 Packages
Atteint http://apt.postgresql.org jessie-pgdg/9.6 amd64 Packages
Ign http://apt.postgresql.org jessie-pgdg/9.6 Translation-fr_FR
Ign http://apt.postgresql.org jessie-pgdg/9.6 Translation-fr
Ign http://apt.postgresql.org jessie-pgdg/9.6 Translation-en
Ign http://apt.postgresql.org jessie-pgdg/main Translation-fr_FR
Ign http://apt.postgresql.org jessie-pgdg/main Translation-fr
Ign http://apt.postgresql.org jessie-pgdg/main Translation-en
Lecture des listes de paquets... Fait

WARNING: apt does not have a stable CLI interface yet. Use with caution in scripts.

linux-image-amd64/oldstable 3.16+63+deb8u1 amd64 [upgradable from: 3.16+63]

#should see also:
linux-image-3.16.0-4-amd64/oldstable-updates 3.16.51-3 amd64 [upgradable from: 3.16.51-2]