Vivian knew about The Plane of Dreams. When Hao Ren returned from seeing Raven 12345, he had told her about the origin of unusual creatures. So, they had their minds on The Plane of Dreams when they heard about The Day of Return.
Even though there was still no concrete evidence pointing to the shadow of The Plane of Dreams behind The Day of Return, as the hometown of all unusual creatures on Earth, it certainly had a defining influence over the creatures worldwide. If the unusual creatures were regaining their power once lost a thousand years ago, then the probability of paranormal activities occurring in The Plane of Dreams would be very high.
If all this was not enough to trigger their concern, then the siren's prophecy delivered by Bluphlei should have been conclusive proof of evidence. To those in the know, there was nothing like the world of strange species and supernatural powers that would befit the ancient 'Glorious Imperium' name.