At the end of Arrival we see a hazy view of Ian looking out over the lake from the view of the inside of the house he shares with Louise. She taps on the window. Then it looks like he gives a slight wave before he turns around to come in the house. Did I imagine that? If not who was he waving to?
Yes that happens. But to get this scene, you need to understand the order of events.
ref - Arrival Explained
The punchline in the movie or the revelation in the climax is that the scenes where Louise Banks (Amy) is shown with her daughter, is not the past. Those are her visions of her own future.
During the events of the Arrival, Louise meets Ian for the very first time. Through the interaction with the aliens and the study of their language, the two of them fall in love. Once the aliens leave, the two admit their love for one another and start their lives together.
That scene where Ian is waving is to one of the officers (perhaps stationed at Louise's place for security reasons). Right after that, he comes inside the house to join Louise. These events are soon after the aliens leave.