I looked over the OP_ZoneSpawns code last night ds, and I'm inclined to believe you.
The 'Bitfields' segment (that started in SoD) has a lot of padding around the 'invis' bit - amongst other changes.
The field is unpacked with an assignment of
Code:
x = (y >> n) & 0xFFF
..so, there could be other needed fields in there.
Trying to see if '08' is SoF or SoD accurate for comparison. (Doesn't look like there are bit-shifts in the unpack..so, probably SoF accurate...)
I'm thinking there is some kind of 'sneak'-like flag being set for all mobs.
When I perma-sneak all out-going packets, the client doesn't make any sounds when moving..but, does otherwise (normally.)
I tried doing a packet dump of out-going server packets for a captured comparison..but, that feature doesn't appear active any longer.
The only other thing I can think of would be that they snuck in a bit/field into the position delta packet and there is a audibility flag there.
Client->Server packets are still dumpable..so, I may be able to confirm yea or nay on that.
EDIT: I did notice with SoF in Steppes that most mobs seemed ok while walking..but, gnolls appeared to have a running sound when they walked.