Pass is the 4th unit action button for Pioneers.  I do not know of a situation where this is not the case (see screenshot)

http://screencast.com/t/msHGnTrpJ

The hotkey (spacebar) can be seen in the tooltip for the action.

Okay, I see what you mean.

Well, you have a few options:
- With the Army selected, SpaceBar will pass.
- You can have them Guard the tile they are on (basically passes their turn every turn until you turn it off).  Hotkey is G.
- You can have them AutoExplore, hotkey is X.
- With the army selected, you can click on the first guy's circle picture in the army context, to see his actions.  Pass button shows up for him.

Hope that helps!