Well, the mob that dropped sneak for me was a yag, so those work too I guess. Oddly enough, I've never had a quad drop a spell for me... But maybe that's because I don't level much around bastok aside from vomp hill. =P
I'd like to get my hands and swords on the assclown who thought highly variable durations from 2 seconds to 10 minutes on this spell was a good idea... Even with High Mnd, I've had it drop in seconds... usually when surrounded by Antica or worse......
I read in Playstation magazine (like 1 year ago, I just remembered), that certain armors made more 'noise' than others, potentially heavier armors. Now I haven't found any confirmation, and it doesn't seem to affect anything, so I can't say. It doesn't really relate to the spell but I just wanted to inquire.
Interesting fact: this drops off low level enemies somewhat frequently, and as a result is sold for a low amount of Gil.
On Bahamut, the price is usually at 300 Gil or so. However, this scroll can be sold to any NPC for (I believe at the lowest) 462 Gil. If you see these going at the AH for 300, buy all you can, and sell them for some quick cash ^_^.
This dropped off a mob in Buburimu Peninsula near the entrance to Mhaura, but I forget which one (either a Goblin or a Bull Dhalmel!). I'm very sure about this cause I had just hit level 20 and bam a level 20 spell fell into my lap. :)