I just now realized that I should have better surprise rules. Something a little more mechanistic than what's in Campaigns. This will overrule some of what I posted a few days ago.
First, roll distance and direction of the encounter if this not already clear. For something keyed to a dungeon room, this is obvious. For wilderness and wandering monsters, roll a 1d and count hexes clockwise for direction. Sometimes you can skip this, and just say whatever it is is up ahead. This is especially so in the wilderness. For distance, roll 4d for yards away and add the biggest SM on each side to the roll (ogres vs. ogres is +2 since the biggest SM on each side, +1, is added twice). This distance is tenfold outside.
Now, everyone rolls Perception. Modify by relative SM (if you're bigger, it's a penalty, if you're smaller, it's a bonus, so Bilbo got a bonus against those trolls), distance on Speed/Range, number in the other party turned into yards on Speed/Range (so 1 foe is a -2, 2 is no penalty or bonus, 3-4 is a +1, 5-6 is a +2, and so on). If the other guy is trying to be sneaky, this becomes a Quick Contest of Stealth vs. Observation instead, same modifiers. If you make the check, you're not surprised. If both sides make it, the encounter happens at the initial distance. If either side is surprised, the distance is 1d yards plus the biggest SM on each side, tenfold outside.
Now we figure who acts from the rules on p. B393. I think its safe to say that adventurers out on a trek are not totally surprised unless they are sleeping, which means that only the guy on watch duty gets a Perception roll, at least at first. This Perception roll for someone asleep would be at -10 or something else ghastly if you want to risk it. In the wild, most foes will be asleep when it isn't their time to be roaming, though smart foes will have watches, just like PCs do.