IceBear said:
Yes, but if that movement towards the surface was a partial charge (and thus an attack) that WOULD be the start of the surprise round, and thus I wouldn't allow the Listen check.
Okay, some thoughts.
1-Gameplay consideration.
Let's assume the PCs happen to walk over the exact spot that allows a rhemoraz to make a partial charge without having to move to get there.
A-You should get a listen check because you should always get a chance to detect an ambush. Ambushes without either a listen/spot check are like a trap without a search/disable device check. They shouldn't exist. They stink of DM whims, they tell to the player: ''I wanted you to be ambushed and that's all there is to it. I decided it would happen and the fact that you decided to play an elven ranger with the alertness feat isn't going to change that.''
B-You should get a listen check because as soon as you start moving you break the statu quo. When the Rhemoraz starts moving, it'll take him anywhere between 1 and 3 second to reach the surface. It's more than enough to justify the possibility of hearing and acting before it's too late.
C-You should get a listen check because contrary to the assassin's example it is your first chance to hear the critter.
2-Common sense.
So you're telling me the Rhemoraz customarily never moves and that the PCs are simply conveniently walking over him?
Random encounters are usually around 5-8% an hour, but if the nasty predators are going to play statue the chances of meeting them should drop drastically. What are the odds you'll walk right over a rhemoraz considering their hunting territory has got to be several square miles wide?
We must assume that Rhemoraz moves around a lot in search of food, probably above ground most of the time. They are huge beast and certainly require a huge amount of food to sustain their metabolism, which seem to be quite fast if they can manage 10D10 of heat without being fire creature!
For example, Tigers travels for miles each day in search of game. It is logically impossible for a monster with the bodymass of a rhemoraz to hunt by staying immobile most of the day hoping for a prey to come by.
This leaves plenty of opportunity for an alert party to notice the presence of the rhemoraz.
I would add that I doubt that the regular diet of the rhemoraz consist of PCs. More likely elks and such compose its main food source. Therefore it should be somewhat unlikely that the PCs walk over a rhemoraz unless they are also in the immediate vicinity of a more common food source.