So, in the orc ambush example, you give characters two potential chances to avoid the danger of surprise? Once (as a group) while travelling and, failing that, then once again (as individuals) when combat starts?
That's what the rules say. The rules tell you, explicitly, that:
- Use the passive Wisdom (Perception) scores of the characters to determine whether anyone in the group notices a hidden threat.
That's a group check, since it mentions the group, and travelling is all about the group (averaging of speeds, etc.). And this is where, of course, the sentence about characters engaged in other activities not contributing to the group check, which is normal.
Note that, if the characters detect a hidden threat in advance (thanks to scouting and probably characters dedicated to that), they might actually avoid it. Or tell them to come out of hiding to discuss, etc.
However, once the combat start, the rules on travelling are again very precise, at the bottom of the paragraph, it says:
- Surprising Foes. If the adventurers encounter a hostile creature or group, the DM determines whether the adventurers or their foes might be surprised when combat erupts.
It's totally separate from the above, it's the individual check on surprise that the combat rules mandate. And this is for absolutely everyone: "The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (
Stealth) checks of anyone hiding with the passive Wisdom (
Perception) score
of each creature on the opposing side."
See also the section on surprise in the SAC: "You can be surprised even if your companions aren’t, and you aren’t surprised if even one of your foes fails to catch you unawares."
