/usr/share/upstart/sessions/hud.conf is in hud 14.10+16.04.20160415-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | description "Unity HUD"
# Currently started by the dbus service file
# start on dbus-activation com.canonical.hud
stop on desktop-end
start on started dbus and ((xsession SESSION=ubuntu-touch) or (xsession SESSION=ubuntu-touch-surfaceflinger) or (xsession SESSION=ubuntu))
exec /usr/lib/x86_64-linux-gnu/hud/hud-service
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