Warning! MAJOR SPOILERS for Star Wars: The Clone Wars season 7 finale ahead.
In Star Wars: The Clone Wars season 7 finale, "Victory and Death", Darth Vader appears in the episode's closing minutes and sees a bird flying in the sky. But it's not just any bird, and its inclusion in this scene references not only Anakin and Ahsoka's past but their future as well. As fans of Star Wars Rebels already know, the former Master and Padawan are destined to meet again, and when they do, it will be not be as friends but as enemies.
The Clone Wars season 7 is the last season of the animated series, finally bridging its story with that of Star Wars: Episode III - Revenge of the Sith. The Skywalker saga is in many ways only now complete, with Clone Wars depicting the final days before Anakin's fall to the dark side. During this time, Anakin meets one last time with Ahsoka, returning her lightsabers, now repaired and tweaked to have a blue rather than green hue. The scene comes just prior to the Siege of Mandalore, and it's a touching reminder of how much they genuinely care about one another. Sadly, it's the last time they are together before Anakin becomes Darth Vader, but even after his downfall, Clone Wars continues reminding viewers of the deep connection shared between them.
In Star Wars: The Clone Wars season 7, episode 12, "Victory and Death", Ahsoka leaves her lightsabers at the crash site of the Republic cruiser. Some time later, Darth Vader arrives and discovers them, picking one up and turning it on. When he does, he senses something, looks up in the sky and sees a convor (an owl-like bird creature) circling above him. This particular convor is green and white and is called Morai. She's appeared before on Star Wars Rebels and is closely associated with both Ahsoka and the Daughter - a powerful light side Force-user who gave her life to save Ahsoka's in The Clone Wars season 3. Vader is likely seeing Morai in the closing minutes of The Clone Wars season 7 finale because it symbolizes Ahsoka is still alive, though Vader probably doesn't realize this it what the convor means.
Morai first shows up in Star Wars Rebels, appearing briefly following a conversation about the Force between Ahsoka and the young Jedi, Ezra Bridger. She's again seen at the Sith temple on Malachor, observing Vader as he limps away and flying after Ahsoka following their duel. Her connection to the Daughter and Ahsoka, however, isn't really explained until Ezra discovers the World Between Worlds.
This Force dimension exists outside normal time and space, and Ezra accesses it through an ancient mural depicting the Daughter, the Son, and the Father - three powerful Force-users introduced in The Clone Wars season 3's Mortis Arc. In those episodes, Anakin, Ahsoka, and Obi-Wan visit the planet Mortis where they meet these three beings. Their experiences there reveal more about the Force and the Chosen One prophecy, but Ahsoka is also killed by the Son, the dark-side opposite of the Daughter. To save Ahsoka, the Daughter channels her life essence into her body, reviving Ahsoka but also ending her own life.
In the mural of the three Mortis Force-users that Ezra discovers, the Daughter is shown with a white and green convor sitting on her shoulder. When Ezra later enters the World Between Worlds, this convor is there to guide him to the portal that allows him to save Ahsoka from perishing in her duel with Vader. Once Ahsoka sees the convor, she recognizes her and calls her a friend, even remarking that she owes the convor her life. Ahsoka eventually goes back through the portal to her correct time and Morai follows, now appearing as she was previously shown, flying after Ahsoka as she descends the steps of the Sith temple.
The connection between Ahsoka and the convor, Morai is pretty clear, and for audiences, any sighting of Morai is definitely meant to remind them of Ahsoka. However, the same cannot necessarily be said for the characters because Ahsoka, Anakin, and Obi-Wan's memories are wiped following the events on Mortis. Not to mention, the link between Morai and the Daughter isn't fully revealed until later on. In which case, when Vader sees the convor circling above in the Star Wars: The Clone Wars series finale, he's unlikely to make the same connection as viewers. But then, it's certainly possible that while holding Ahsoka's lightsaber and seeing the convor overhead, Vader has some sense that his Padawan is still alive.