I thought I recalled it being in the levels where you run around between fancy subway stations and down until subway platforms and terminals, but I didn't find it on this playthrough. Anybody know?

That's where I seem to remember seeing it. It is kinda small though, and if you already have all your weapon slots full, you won't pick it up if you just happen to run over it.

I recall one of the Zealots drops it in other "museum" section but it's kind of random, small and too dark to see it clearly. Never found them drop it again, only by glitch.

Which game?

Blood2 obviously, without mods. It's not just on the floor until one of these guys are killed but since it's so dark I miss it too often. This late museum connected with subway is at least revisited twice, but only one chance to get the Orb first time. I think monsters spawning certain items are just like first game - not random, predetermined on maps, otherwise all Zealots should spawn the Orb and all Drudge Lord should spawn Life Seed. ;p

And how is that obvious?

It's not particularly, only if you happen to know which weapons are different from Blood to Blood II.