Comment 7 for bug 1505337

Speaking to mvo about this he believes its possible that update-manager and aptdaemon were updated but that the running aptdaemon was not replaced with the new version of aptdaemon. Subsequently, the flag was passed to aptdaemon and it didn't know how to handle it.

Looking at the Aptdaemon.txt attachment in this bug report we can see the same process id used for aptdaemon even after the package is upgraded.

Log started: 2015-10-12 11:10:01
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(Reading database ... 870936 files and directories currently installed.)
Preparing to unpack .../python3-aptdaemon.pkcompat_1.1.1+bzr982-0ubuntu13_all.deb ...
Unpacking python3-aptdaemon.pkcompat (1.1.1+bzr982-0ubuntu13) over (1.1.1+bzr982-0ubuntu12) ...
Preparing to unpack .../python3-aptdaemon.gtk3widgets_1.1.1+bzr982-0ubuntu13_all.deb ...
Unpacking python3-aptdaemon.gtk3widgets (1.1.1+bzr982-0ubuntu13) over (1.1.1+bzr982-0ubuntu12) ...
Preparing to unpack .../aptdaemon_1.1.1+bzr982-0ubuntu13_all.deb ...
Unpacking aptdaemon (1.1.1+bzr982-0ubuntu13) over (1.1.1+bzr982-0ubuntu12) ...
Preparing to unpack .../python3-aptdaemon_1.1.1+bzr982-0ubuntu13_all.deb ...
Unpacking python3-aptdaemon (1.1.1+bzr982-0ubuntu13) over (1.1.1+bzr982-0ubuntu12) ...
Processing triggers for dbus (1.10.0-1ubuntu1) ...
Processing triggers for man-db (2.7.3-1) ...
Setting up python3-aptdaemon (1.1.1+bzr982-0ubuntu13) ...
Setting up python3-aptdaemon.pkcompat (1.1.1+bzr982-0ubuntu13) ...
Setting up python3-aptdaemon.gtk3widgets (1.1.1+bzr982-0ubuntu13) ...
Setting up aptdaemon (1.1.1+bzr982-0ubuntu13) ...
Log ended: 2015-10-12 11:10:06

Oct 12 11:09:23 impulse AptDaemon[7097]: INFO: Initializing daemon
Oct 12 11:09:23 impulse org.debian.apt[1349]: 11:09:23 AptDaemon [INFO]: Initializing daemon
Oct 12 11:09:23 impulse AptDaemon.PackageKit[7097]: INFO: Initializing PackageKit compat layer
Oct 12 11:09:23 impulse org.debian.apt[1349]: 11:09:23 AptDaemon.PackageKit [INFO]: Initializing PackageKit compat layer
Oct 12 11:09:23 impulse AptDaemon[7097]: INFO: UpdateCache() was called
Oct 12 11:09:23 impulse org.debian.apt[1349]: 11:09:23 AptDaemon [INFO]: UpdateCache() was called
...........
Oct 12 11:12:45 impulse org.debian.apt[1349]: 11:12:45 AptDaemon.Worker [INFO]: Updating cache
Oct 12 11:12:49 impulse AptDaemon.Worker[7097]: INFO: Finished transaction /org/debian/apt/transaction/458231a0ba3346518ce0f4c60fff50ee
Oct 12 11:12:49 impulse org.debian.apt[1349]: 11:12:49 AptDaemon.Worker [INFO]: Finished transaction /org/debian/apt/transaction/458231a0ba3346518ce0f4c60fff50ee
Oct 12 11:12:57 impulse AptDaemon[7097]: INFO: CommitPackages() was called: dbus.Array([dbus.String('kinit#auto'), ...

So this theory seems likely.