The kind of thing you are talking about is quite common. The verb can be dropped when it is well understood what the assumed action would be.
So in your example, it says
「部屋に」【へやに】, "to the room". Note that "room" in Japanese often stands in for "apartment", or some understood location.
The key is in the particle,
に, which indicates an action toward the thing it is pointing at, which in this case is a room. One doesn't do many things as an action toward a room, other than go to it, so we can assume the missing verb is something like
You might wonder if it's come or go, and there might be hints in the context, but either way we end up in the room, so everyone's happy. So the translation is something like "As soon as that guy returns, I think we'll head to/go to/come to the room."
Maybe you could jump into the room through the window, so might the missing verb not be something like
飛ぶ【とぶ】(fly)? The key is that when you hear galloping, think horses, not zebras. The reason the verb is dropped is because it's the obvious one.
For further consideration, imagine if the sentence ended with
を indicates that the room is being acted upon, so something is being done to it. That would be weird. Not that sentences can't end in
で or whatever else, it's just that the rest of your example sentence doesn't really have enough meat on it to support
を, because I can't think of an obvious action on a room.
Maybe it could be done if there was a whole context of conversation about, I don't know, blowing the room up, and so everyone knew the missing verb was
「吹き飛ばせる」【ふきとばせる】(to blow something up).
In any case, verbs, like many things in Japanese, are dropped when the speaker or author thinks the listener or reader will be able to roll along without it.
Lastly, for an extreme example, I had an experience once where I asked a female friend what she did earlier in the day, and she replied
「バーゲン」, "bargain", meaning that she had been to a bargain sale. I teased her for not using any verbs, but still, there you have it. You can get away with just a noun and not even a particle.
Hope that helps.