mmm - sometimes it can be context. The people in the other thread called "car up" because they were going to be entering a one lane bridge and there was already a car there.

When I'm racing, sometimes we call car up, just to warn those who are near the center line to be careful.... (but in general group rides should not be spread out over the road like that....)

Sometimes you'll hear people on a regular group ride call car up if the group will be turning and they want to warn everyone to not turn in front of a car.