Ok, got it:
2019-10-31 06:37:51,377 DEBUG [4740:Dummy-22] mopidy.audio.gst: Got about-to-finish event.
2019-10-31 06:37:51,377 DEBUG [4740:Dummy-22] mopidy.audio.actor: Running about-to-finish callback.
2019-10-31 06:37:51,378 DEBUG [4740:LocalBackend-7] mopidy.backend: Backend translated URI from local:track:arbs/musikk/talking_heads_-_once_in_a_lifetime.mp3 to file:///data/audio/arbs/musikk/talking_heads_-_once_in_a_lifetime.mp3
2019-10-31 06:37:51,382 DEBUG [4740:Dummy-22] mopidy.audio.gst: Got source-setup signal: element=GstFileSrc
2019-10-31 06:37:51,383 DEBUG [4740:MainThread] mopidy.audio.gst: Got ERROR bus message: error=u'gst-resource-error-quark: Resource not found. (3)' debug=u'gstfilesrc.c(533): gst_file_src_start (): /GstPlayBin:playbin0/GstURIDecodeBin:uridecodebin0/GstFileSrc:source:\nNo such file "/data/audio/arbs/musikk/talking_heads_-_once_in_a_lifetime.mp3"'
2019-10-31 06:37:51,383 ERROR [4740:MainThread] mopidy.audio.gst: GStreamer error: gst-resource-error-quark: Resource not found. (3)
2019-10-31 06:37:51,402 DEBUG [4740:MainThread] mopidy.audio.gst: Changing state to GST_STATE_NULL: result=GST_STATE_CHANGE_SUCCESS
Mopidy is now stuck waiting to finish playing the previous song.
EDIT: Oh, I see this is in fact a file that has been removed since the last time it was played. So it’s basically a file that’s not found. But why does mopidy stop playing when this happens? It should just move on?