If I had to bet I think Imu that lunatic took matters in his own hand and just sent his Ancient Weapon over there.
If you think about it: According to the spoilers Mars already flies up to Punk Records (the very top of Egghead island) and even then looks even further up.
We already saw the Ancient Robot - while tall it is WAY smaller and doesn't reach up there at all. Mars would have to look down to spot it.
Dragon is obviously nonsense, he was still chilling on Kamabakke island a few minutes (in-manga time) ago.
Zoro and Sanji being that high up in the air (top of Egghead island) all of a sudden makes no sense at all. They would have to pass the barrier first btw and they can't do that.
Kizaru is the only other option that makes remotely sense, but it's a very bold guess to say the least.
Wildcard would be Big News Morgans arriving with his flying ship.