A few of the Horrors managed to sneak past the sentries on at least three occasions. Once durng the Great Ghost Dances (each one could be considered separate, but the big one was with the volcanoes), once when Aina nuked Ysrthgrathe (and he thanked her for it, the bastich), and once when Darke formed his bridge to their Netherworld. I don't think any came out when Ghostwalker returned (it's hard to call him that, to me he will always be Icewing) cause I'm pretty sure the dragon would have obliterated or evaded them before coming through.
That leaves it open for a number of minor Horrors to spread out, at the least. Wraiths showing up is a sign of their (eventual) return, but I believe that most of the doorways to their realm have been sealed by Billy and Lethe. Still, a few stories with the Enemy may creep in once in a while.