I have also never encountered adventurer parties while playing OOO or MMM. Maybe that is FCOM? I wouldn't know though I never took the FCOM plunge yet.
I may have an ask in the FCOM thread about that. I would really like to see that if it exists. Does anyone know possibly how common the chances are to see either the adventurer parties or the other NPC's added to dungeons from OOO?
Can't give a percentage of the chances as I haven't checked, but it's not uncommon so a bit odd if people using OOO and/or MMM or FCOM haven't stumbled upon them:
OOO:
Adventurers
Adventurers regularly explore dungeons, and they may also loot clean dungeon containers. If the containers are locked, then the adventurers will use their lockpicks to open them. If they run out of lockpicks, they will not be able to open the container. Finally, OOO traps now affect adventurers as well. You now have the dilemma of whether to let adventurers loot containers or not. They could get rid of a trap for you, and receive the damage themselves, but by the same token, they could survive and take whatever was found inside the container.
MMM:
Adventurer Overhaul::
The Adventurers in MMM have undergone an overhaul also, bringing the total number to at least 55 different Adventurer types you can meet and include everything from Scouts and Bards through Plunderers, Tomb Raiders, and Warriors.
Additionally, equipped armors have been split into Light and Heavy for the various classes, there's a greatly reduced chance of carrying magical items, greatly expanded personality/background loot drops, and stealthier classes now sneak while looting. Finally the two biggest changes include: Adventurers now can loot chests from both OOO and Frans and, best of all -- you can now find them in Oblivion too, in larger numbers than in dungeons and ruins. Like you, they're risking an assault into Oblivion itself.