I suspect xfce4-power-manager and/or UPower and/or ACPID and/or systemd may all be trying to decide that the N900 needs to be shut down real quick because they may believe that the battery is empty (looking at wrong /sys path or whatever). This is only a suspicion. For some reason whoever/whatever is aggresive enough to decide to shutdown the N900 is nevertheless too shy to drop something on the syslog. Shameful behaviour, really.