Yes, there is sadness, particularly when someone is conspicuously absent as opposed to being coincidentally missing. And I suppose this sort of absence/ loss would be a bit bittersweet for someone who's more than aware that the absent party is normally present. Just my thoughts on your thoughts.
Well I dont know, I feel like it made me confused only because you said "presence of absence" almost a complete contradiction. Although I could generally say that this whole piece seemed "nonsensical" no offense, but theres a hole in what crowd, I do get the invisible reference and it somewhat makes sense but its probably too confusing for the average reader to take in, maybe make it longer to express the intent of the poem.