I could be wrong but I think they will appear if you target an enemy rather than just when you engage them.
However note the cautionary tale of Taugrim in his Gunslinger PVP guide, in that the green gumby does not always put you in a favourable position.
Yes, Tab targeting nearby enemies will show you the green gumby. And while it is true that auto-rolling into one might not always put you where you want to be (quite often been directed 90 degrees towards the enemies I want to fight, giving me no protection against most of them).
, note that the green gumby's are merely suggestions! You can get full cover bonuses from most things that look like they'd give cover bonuses, by merely standing next to them, and crouching. You DO NOT need to roll into a green gumby spot to enjoy the defensive bonuses!