Yeah, there is NO WAY the Alberto we saw in Istanbul has been voluntarily staying away from Ana for years. Months, yes. Years, no. Especially since in those years he would HAVE to have seen/heard of Ana becoming a success -- and with her fame, surely the press would be circulating the story of how she dealt with the tragic "death" of the love of her life.
I can come up with only three possibilities:
1. Amnesia. That is, while on his travels, he had some sort of accident and doesn't remember who he is, so he's made some other life for himself in the five years since we last saw him. Then, somehow, Cristina finds out about it and has to decide what to do.
2. He's trapped somewhere. He was in Istanbul. Who knows where he went next or what trouble he may have gotten into? He could be in jail. He could be trapped in a foreign country with no way to get back into Spain.
3. Sometime after we saw him he had a change of heart about returning to Madrid. This is the least plausible, but I can't rule it out. Here's the scenario: At some point Alberto DOES return to Madrid, but before he learns that they all think he's dead, he sees Ana/Carlos out with a baby and assumes it's Carlos's child and Ana has moved on (which, he believes, is why she never answered his letters).
During this time, he also learns that Ana is an international success and that Velvet is doing well without him. Totally dejected, he turns his back on Madrid, deciding that Ana and Velvet are better off without him. He doesn't even contact Mateo because it's all too painful and maybe he worries that Mateo would try to talk him into staying.
So he leaves and decides to start fresh somewhere else. He ignores all news of Ana, and thus never learns that everyone believes he's dead. When Cristina finds him, he's super bitter and she has to decide whether or not to tell him the truth.
What do you guys think? Which one sounds more plausible?