He watched it dissipate. The first tatters followed the prevailing winds, tugging the cloud into an ovoid, delicate cotton-candy streamers drifting from its downstream end. After a few moments, though, it began to disperse laterally as well, its component particles instinctively sniffing the air for signs of treasure.

No obvious movement against the wind. That would have been too much to hope for, this early in the game.

He fired the next grenade a hundred meters diagonally upwind of the first; the third, a hundred meters from each of the others, the closing point of a roughly equilateral triangle. He zigzagged his way across the wasted landscape, kicking little drifts of ash where bracken and shrubbery had clustered a day earlier, navigating endless rocky moguls and fissures. Once he even hopped across a scorched streambed, still trickling, fed by some miraculous source further upstream than the flamethrowers had reached. At rough, regular intervals he shot another absurd pink cloud into the sky, and watched it spread, and moved on.

He was aiming his eighth grenade when he noticed the residue of the seventh behaving strangely. It had started as puffy round cumulus, like all the others. Now, though, it was streaked and streaming, as though being stretched by the wind. Which would indeed have been the case, if it had been streaming with the breeze instead of across it.

And another cloud, more distant and dissipated, seemed to be breaking the same rules. They didn't flow, these aerosol streams, not to the naked eye. Rather, they seemed to drift against the wind, towards some point of convergence back the way Lubin had come, about thirty degrees off his own track.

And they were losing altitude.

He started after them. The motes in those clouds couldn't be called intelligent by any stretch of the word, but they knew what they liked and they had the means to get to it. They were olfactory creatures, and they loved the smell of two things above all else. The first was the protein signatures put out by a wide array of weaponised biosols; they tracked that aroma like sharks sniffing blood in the water, and when they finally found that ambrosia and rolled around in it they changed, chemically. That was the other thing these creatures loved: the smell of their own kind, fulfilled.

It was the classic biomagnifying one-two punch. Too often, traces of one's quarry were too faint to do more than whisper to a few passing motes. Those would lock on, enzyme-to-substrate, and achieve their own personal nirvana — but that very merger would quench the emissions that had lured them in the first place. The contaminant would be flagged, but the flag would be far too small to catch any mammalian eye.