When I call for observation, I call for: Observation/ sense sight (or some other relevant sense), Sense ambush/ awareness, or surveillance (which is more like observing an area for a longer time, say 5 minutes)
Using the individually developed senses, you wouldn't be subjected to the "general penalty" of -20, which you would get with observation if it pertains to senses other than sight, so sense sight = observation, otherwise use relevant sense, or observation -20.
The sense ambush/ awareness roll is for detecting imminent danger. That is why they both have limited development.
The surveillance roll is for staking out a place, or guarding a place, or following a subject that is unaware of your interest.