When I start a service on a system bus:
# systemctl --system start apkd.service
dbus-monitor --system "type='signal'"
signal sender=:1.1 -> dest=(null destination) serial=527 path=/org/freedesktop/systemd1/unit/apkd_2eservice; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged
But when I start a service on session bus:
$ systemctl --user start pebbled.service
I see nothing on:
$ dbus-monitor --session "type='signal'"
"Note that most signals are sent out only after Subscribe() has been invoked by at least one client."