Actually thinking more on the idea that he escaped on a motorbike. I have to think on two things.
1 - Where did the motorbike come from? I'm pretty sure demons don't drive on motorbikes. Unless that's something they want to put into DmC, I don't know! But I know it never happened in DMC. So where did the motorbike come from if Dante was stuck in Hell?
2 - I'm quite sure that Dumary Island is kind of like Mallet Island. Islands have water surrounding them, which is why they give them the name 'Island'. As far as I'm aware, you can't ride a motorbike on WATER!
And then we have to think about who it could be, logically. Dante is stuck in Hell (So it's unlikely that it's him). It can't be Lucia because she is in Dante's office. So that leaves us wondering who it could be that we would know about. Well, DMC 2 IS set after Devil May Cry 4. So it leaves us to wonder...
1 - It was Trish, because she has been close partners with Dante for a while now. Longer than Lady or Nero would of been allies to him, so they have more history together. I would also imagine that Trish may of needed Dante's help in a job of her own. If we recall in the DMC Anime, also set before DMC 2, the pair split up and went their own separate ways. However, they still pair up when necessary (Devil May Cry 4 is proof of that). So maybe Trish needed Dante's help and came to his office seeking it.
2 - It was Lady. Just like Trish, we know she rides on motorbikes (Motorbikes seem to be a favourite vehicle for Capcom). Lady is the one who gives people 'odd jobs' and 'missions'. Perhaps she came to Dante's office to give him a job of some sort. She would ultimately be going there to get ahold of Dante, as she doesn't know who Lucia is yet. And she wasn't the one to of given him the mission to go to Dumary Island, nor was Trish. So maybe something else came up. I can't see it likely to be anything of great importance. It's unlikely that something just as big as Argosax is going to crop up at exactly the same time.
3 - It was Nero. Okay, so Nero was never probably thought up back then when they were making DMC 2. BUT, you can't disregard him from potentially being part of the story. We never see the character on the motorbike. So we can only make assumptions. Dante seems to leave Nero behind in Devil May Cry 4, but he asks if he will ever see him again (Kind of hinting that he probably will do). Nero would be more powerful to enter into Hell and help 'rescue' Dante if need be. More useful than Lady or Trish. And even more useful and more powerful compared to Lucia. So although far-fetched, it's still plausible.
...
4 - It was Vergil. You heard me right. A lot of unexplained things happened surrounding Devil May Cry 4. All to do with who Nero was, his background and the like. And Vergil apparently seems to play a big part in the 'theories' regardless to him being in Devil May Cry 4. And Vergil being presumed dead. Although as we all know, Vergil is classed as being 'defeated'. Vergil was never seen dying, he was seen 'leaving'. Whatever happened to him afterwards is a mystery to everyone. But I'm quite sure he didn't die. Although personally even if I am saying it 'could' of been Vergil on the bike, I'm seeing that as a very slim chance (No matter how cool it would of been). Vergil's motives are never truly known, so I couldn't speculate as to why he would of shown up at Dante's office (If it was even him).
