Not sure how to trigger, but happens rather frequently. See attached screenshot.
DISTRIB_ID="elementary" DISTRIB_RELEASE=0.4 DISTRIB_CODENAME=loki DISTRIB_DESCRIPTION="elementary OS 0.4 Loki"
$ wingpanel [INFO 19:37:28.636257] Application.vala:155: System Panel version: 2.0.1 [INFO 19:37:28.636287] Application.vala:157: Kernel version: 4.4.0-64-generic $
$ # let's kill it $ killall wingpanel $ # icon gone
Bug watches keep track of this bug in other bug trackers.
DISTRIB_ ID="elementary" CODENAME= loki DESCRIPTION= "elementary OS 0.4 Loki"
DISTRIB_RELEASE=0.4
DISTRIB_
DISTRIB_
$ wingpanel vala:155: System Panel version: 2.0.1 vala:157: Kernel version: 4.4.0-64-generic
[INFO 19:37:28.636257] Application.
[INFO 19:37:28.636287] Application.
$
$ # let's kill it
$ killall wingpanel
$ # icon gone