Sleeks, and most limited wands, can't be seeing via peek whether or not you're with the thief. If you identify them, it'll say something like the item is easily concealed, which also means you can't find it with locate object. I haven't played a thief in over a decade now, but I'm fairly certain that you can't steal sleeks for the same reason.

As for when the sleek spawns.. I feel like it spawns on the corpse only with you in the room. I can't recall ever finding a sleek that wasn't mine in the corpse of a mob I killed, while such a thing does occur on container drops (oblivious or busy mage walks past his container location and doesn't pick it up, leaving the rod there for an unknown amount of time).

In either case (container or mobile drop), you need to be in the specific room rather than just the area for the rod to spawn. On a mob, a rod will spawn if either you or a groupmate lands the killing blow. If your groupmate is also a mage, and the location is also his, whoever lands the killing blow gets the associated cooldown (IIRC) on spawning a rod, so collecting sleeks with a mage partner won't screw someone over for 20-30 ticks.