We were there for 21 nights last year (went to say this year, keep forgetting it's a new year). We had a savannah view room and as my DS loved animals we spent a lot of time on our balcony enjoying our view.
So on the Arusha (can't remember the spelling) savannah that we were on, the animals are out all night, then in the mornings, they are rounded up and put (well most didn't need to be rounded up, it was funny, we loved watching them make their pilgrimage over to the gate of the enclosure they went into for a couple of hours in the morning, where they would wait for the staff to come and open and let them in). They would walk in a line, over there, all except the cattle. They would pose a challenge to the vehicle with the staff who would open the gate to the enclosure. All of the other animals would just walk in, all except the cattle. They would stay right up the top mooing and mooing, and eventually would mosy on down to the enclosure.
Once all in, the vehicle would go around and I assume clean up the savannah.
So depending on what time you went for breakfast, if you went out to the viewing area near the reception, that is the Arusha savannah and all of the animals may have already made their way round to this enclosure around the back of the savannah (that we could see from our room) and been waiting there, or they may have already been put in there.