"One evening [Isaac](rc://en/tw/dict/bible/names/isaac) went out to the field to think." This must have been a long time after the servant and [Rebekah](rc://en/tw/dict/bible/names/rebekah) left her home since they had to travel a long distance.
The word "behold" here alerts us to pay attention to the surprising information that follows. "When he looked up he was surprised to see camels coming"