To resolve this example there are basically four options:
1) passive sense motive, DM rolls and let's only the players whose characters notice something, know. The bard will be the only one who succeeded and so the others start heading west, but the bard might be able to stop them.
2) active sense motive, players don't ask for a roll. All characters head west, the clue hasn't been noticed, regardless of the characters abilities.
3) active sense motive, some players ask for a roll. Either the other players will then also ask for a sense motive roll, or they will simply accept the outcome (maybe their characters share some sort of hive mind, after all

). The result will then be either the same as 1) or 2), depending on who rolled. Most likely it's like 1), since the bard - with the decent sense motive - will most likely be the one that asked for a roll, or will do so, once someone else asks for a roll. In the end, only prescious time has been wasted, while some players asked for a sense motive roll, then others followed suit, or the clue has been missed, if the right players didn't ask.
4) active sense motive, all players ask for a roll. Same result as 1), just that the players had to ask, instead of the roll being done automatically. This result will obviously not always happen, since it's possible for the players not to ask, in which case the same as in 2) would happen.
Only the passive roll leads to consistent results.
Note, that instead of doing all rolls, you can also simply use only the highest skill rating and roll for it, possibly assigning an automatic bonus for aid another, if you want to do it in one roll. Instead of doing it secretly, you could also ask the players to do the roll, but that will just tip them off and might not lead to the same result. Of course, if the players do not metagame, it's no problem. The only point here is, that the roll should always happen, not only if the players care to ask for one.
Bye
Thanee