With both monitors connected, could you please post the output of
$ xrandr
$ gsettings list-recursively org.onboard.icon-palette
$ gsettings get org.onboard.window force-to-top
It might be that the second monitor is connected left of the built-in screen. This would move the icon out of view when you disconnect and the desktop origin jumps by the size of the HDMI monitor.
You'll probably get the icon back with
$ gsettings reset-recursively org.onboard.icon-palette.landscape
With both monitors connected, could you please post the output of icon-palette
$ xrandr
$ gsettings list-recursively org.onboard.
$ gsettings get org.onboard.window force-to-top
It might be that the second monitor is connected left of the built-in screen. This would move the icon out of view when you disconnect and the desktop origin jumps by the size of the HDMI monitor.
You'll probably get the icon back with icon-palette. landscape
$ gsettings reset-recursively org.onboard.