While fan translations may have interpreted this scene as Mickey saying that he saw Silver in the prefect’s bedroom, the EN-server translation is correct: Mickey saw Silver in his dream room, not in Ramshackle.
EN-Mickey says “my dream room” and originally he says “this dream room,” which is very important.
Much like English, Japanese differentiates between things close to/belonging to you (this, この/kono) vs. things close/belonging to other people (that, その/sono), but unlike English the difference is a little stricter.
Mickey uses この/kono for “this room,” which can only mean the room where he is. If he had meant the prefect’s room, he would have said その/sono, for “that room.”
Since English can be more flexible with “this” and “that,” it is possible that translators saw “this room” in Japanese and interpreted it as what “this room” meant to the prefect, not what it meant to Mickey.
From a Japanese-language perspective, however, “this room” is the one occupied by the speaker (Mickey), not the one occupied by the listener (the prefect).
The idea that Silver was in the prefect’s room is further debunked by Mickey’s follow-up line, when he says that he wishes the prefect could also come to see him on “this side” (こっち側/kocchigawa), just like Silver did.
Twisted Wonderland Reference Note 