This is always the case. (3-4)
At this point in the poem, the speaker has already told us he's the absence of field. That idea is strange enough on its own, but lines 3-4 create a sense of dissatisfaction with being absence. In one sense, his matter-of-fact tone could sound like he's totally okay with being absence, but there's also an underlying irritation that no matter where he goes, he can't really be there.
Line 3 breaks on the word "always" which emphasizes the finality of his dilemma of embodying absence. It's like he's saying, "I'm nothing and it's always like that, no matter where I go." We could even go as far as to say line 4, "[t]he case" works as both the situation our speaker finds himself in, but also like a "closed case" (think a closed briefcase, a safe, or a court case where judgment has been delivered). There's no escaping his absence; it's a done deal. And yet, he seems dissatisfied with the results and must carry on, forever caught between existing and not existing at the same time.
and always the air moves into fill the spaceswhere my body's been. (10-13)
Our speaker is so full of absence, even the air chases him away. Or gets out of his way. Or refuses to mix with his kind of absence. Although he could just be reporting the facts of his predicament, he also sounds downright frustrated. Notice how he ends line 10 with "always" just like he ended line 4 with "always." It's a repetition of finality, as if there's no way out. No matter where he goes, even the air won't have anything to do with him. That could be frustrating. Imagine that no matter where you go, nobody acknowledges your presence. Could be fun sometimes, but after a while, if that always happened, you might begin to feel dissatisfied with your isolation from the world.
Lines 12 and 13 make it sound like the speaker's presence is erased by air. Imagine a bad break up and then throwing away everything that reminds you of your ex. You might want anything and everything that reminds you of that person to disappear. Well, it's like the air broke up with the speaker, and the air doesn't want anything to do with him anymore. Even when he leaves, the air rushed in to wipe out "the spaces / where [his] body's been."
I move To keep things whole. (16-17)
Okay, our speaker doesn't come out and say, "I hate having to move all the time!" That would sound like dissatisfaction, and for a guy who's so plainspoken, you'd expect him to be more forthright about his emotions. But by the end of the poem, he's given us several images of how he doesn't exist in the world. His only solution, then, is to move. It's not as if he's headed somewhere to find the love of his life. He's trapped in a constant cycle of being somewhere and not being there, and his only reprieve from that prison of being absence is to keep moving. That could be a wee bit frustrating.
Imagine you stop to take a break, catch a view of the sunset, say hello to a friend, and an overwhelming sense of isolation rises inside of you. Furthermore, the only way to stop that feeling of isolation is to keep moving along so you can escape that feeling…except that no matter where you go, that feeling will hunt you down. The speaker's subtle way of embodying emptiness and isolation is what makes this poem so haunting. Not only is his frustration captured with such simple language, but he ends the poem by saying that his own only refuge is to keep moving so that the world remains whole. It's almost like he feels like the world doesn't want him around, and yet his movement through the world is what "keep[s] things whole." Case closed.