S. R. Faruqi:

In this verse there's an extraordinarily mysterious mood. For dust to arise from caravans that had previously departed has two opposite interpretations. The first is that they became erased and turned to dust, and their dust is flying in every direction. The second interpretation is that they went at such a speed that their dust rose up and spread far and wide, to such an extent that it reached us.

Now, on this occasion, the speaker mentions his own dust. Was its visible presence mingled with that of the caravan, and now that the caravan has passed from view he is waiting for his own dust to rise? That is, has he already become erased to such an extent, and in such a way, that now he is not even dust, but only an invisible presence? --The kind that is mentioned in this verse of Zeb Ghori's:

ek jho;Nkaa havaa kaa aayaa zeb
aur phir mai;N ;Gubaar bhii nah rahaa

[one gust of wind came, Zeb,
and then I did not remain even dust]

But if this is the case, then why does the invisible presence of the speaker worry so much about his dust? He has after all become erased; now whether his dust does or doesn't arise, why should he, or anyone, care? Perhaps because the dying one even then loves his own visible existence, and he wants to see some sign, or some glimmer, of it.

In the second line he has expressed a waiting, and a longing, so excellently that one feels no need of the first line-- although without the first line, that horizon-wide perspective would not have been created, such that whatever caravan has gone on, either it has become dust or else its dust is rising high.

The speaker's visible existence is separate from that horizon-wide perspective; this is its melancholy. Without this retrospective melancholy, the verse would have remained on the level merely of human longing. Despite saying all this, the verse doesn't reveal itself entirely.

In the sixth divan he has composed this same theme clearly [{1911,2}]:

gaye un qaafilo;N se bhii u;Thii gard
hamaarii ;xaak kyaa jaane kahaa;N hai

[from those departed caravans too, dust arose--
who knows where our dust is]

FWP:

SETS
MOTIFS == 'DEAD LOVER SPEAKS'
NAMES
TERMS == MOOD

SRF reads this verse as unambiguously spoken by a dead lover. But the alternative reading he sets up for the departed caravans-- that they were moving swiftly along and thus kicked up a great deal of dust in the course of their journey-- surely could work for the speaker as well. He after all might be moving too, and he might be following the tracks of the giant caravans by watching their huge dust-clouds, and regretting that his small restless dust-cloud seems to be thoroughly obscured by theirs.

Or else the dust of the caravans' passing is a metaphorical expression of their activity, their visible life. They make a big impression on the world-- but what of the private, half-crazed pursuits of the lover? Everyone notices big caravans. How many people notice the solitary, restless, doomed lover? No doubt he avoids the well-traveled trade routes and wanders in the desert, but even there he can't escape the obscuring effects of such massive clouds of dust.

Still, the 'dead lover speaks' reading is poignant in a way that the annoyance (and petulance?) of the living lover is not.

There's also the nice wordplay of gaye and u;Thii (in the sense of 'rose and departed'), versus rahaa -- which itself is called into question in the course of the second line.