Not sure if it documented anywhere in the manual but it looks like it is using icon ID of
LAYER_USED. That is the same icon shown in the 3dview header to show that something is on that layer.
A popup menu normally has two arrows but the cycles materials where this is shown are used for node links and line 1380 of interface_widgets.c supports that theory. What the if statement surrounding that line says is if it is drawing a menu and it is flagged as a node link it draws the layer used icon which elsewhere would also prevent the two little arrows being drawn, that matches with the cycles material display.
LAYER_USED can be found is in the addons - Layer Manager, Node Wrangler and PowerLib.
It would appear that the use you have shown is unique to the cycles material display as part of a popup menu and is hard coded that way. From an addon usage we don't get direct access to show it as part of a popup menu, I haven't tried but you may be able to flag the menu as a node link to make it work.