Pause. An envelope slides under the door, right. GUS sees it. He stands, looking at it.
This is not as hidden a "space" as the hidden dumbwaiter or perhaps the lavatory but I do believe it is important to the function of this story all the same. We read that an envelope has been slidden from under the RIGHT door, and that envelope contains matches. There is no note that accompanies the envelope, there is no trace of a person that left the envelope. There is only the envelope with the matches. This envelope is creepy because we are in the dark about who gave it to Gus and Ben -- and why did they give it to them? Through this whole play, there are dark, strange forces operating outside of Gus and Ben's view. There are assumedly people or beings operating the dumbwaiter and sliding envelopes yet we only ever see Gus and Ben. I also think it's incredibly important to note that the envelope is retrieved by GUS from the RIGHT door. Because at the end of the play, Gus enters in from the right door (the first time he's done this). So to me, the matches feel like a premonition of what's to come -- of Gus's eventual demise at the hands of his partner.