When seahorse-daemon is not autostarted by the session, it is autostarted by D-Bus the first time it is called. In this case, the first D-Bus call always fails, while subsequent ones work. This can be reproduced easily with any of the clients in seahorse-plugins, which show this kind of error:

** (gedit:10626): WARNING **: dbus call to list keys failed: Launch helper exited with unknown return code 0

This is because seahorse-daemon, as the name says, daemonizes itself upon startup. Here are the comments of our D-Bus maintainer:

 <Np237> hmmmm this might be because seahorse-daemon daemonizes itself
 <Np237> I don’t know how daemons started by dbus are supposed to behave
 <sjoerd> Np237: they shouldn't daemonize themselves
 <sjoerd> Np237: if it first daemonizes itself and only then connects to the bus, the launch helper doesn't know wtf just happened
 <sjoerd> other then the thing it just launched did an exit(0) before it could actually call dbus methods on it
 <sjoerd> it would also explain prefectly why it works the second time
 <Np237> ok, so that’s clearly the explanation for seahorse-daemon
 <Np237> sjoerd: will D-Bus itself daemonize it correctly?
 <sjoerd> yeah