Ambience mode player here, here's what I noticed after 40 days:
Skerfur can trigger in ambience. This happened to me when I sent my Kerfur-O outside around 1AM. This seems almost certainly like a bug. As the Skerfur event is pretty spooky.
Superfog can trigger in ambience. Not sure if this is a bug since it's not exactly a scary event. But it does lock out sleeping/pausing/saving during it.
Dreams can trigger in ambience. Not sure if it's working as intended or a bug. Not the most scary stuff but a few of them, like the rising water level or being chased by shadowy figures, that gave me cold sweats.
Toyota Corolla can spawn in ambience. Not the most spooky thing, but I think it counts as one of the random events? Therefore I'm thinking it shouldn't trigger.
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The version I'm playing is 0.8.2c