Good question. I assume it doesn't spring until there's a viable target, and a Gold unit certainly wouldn't qualify. That said, I don't think I've seen a single Ambush unit played since Open Beta. They really over-nerfed the mechanic.
She triggers before the lock so she can end up killing a lot of the faction specific lock units that have less than 5 STR (actually she only doesn't kill Cleaver and Ciaran). I don't know if this was intended or not tho.
Morenn doesn't flip over when your opponent plays a gold. She only activates when a bronze or silver unit is played.
She will also flip and do her damage before the played bronze or silver unit activates their ability so, for example; she will actually activate and kill off a wild hunt hound before it has the chance to activate biting frost.