Sanji, was the 5th member of the crew and a world class chef.
He joined at the end of the Baratie Arc leaving his friends and mentor Zeff back in East Blue. However, Sanji does not originate from East Blue, (the first of the Straw Hats to not come from the “Weakest Sea”) *See S1 (look at the panel below)

The furthest we have seen intro in Sanji’s past was when he was a kitchen boy on a ship that was attacked, and he was saved by Zeff (S2). What we do not know, is how he ended up on the ship and why, (particularly at that age). This is especially puzzling as Sanji is from North Blue, and one would ask how a child made it there. We may say that Sanji is of Noble descent and not just Noble, but Sanji has the Blood of a Celestial Dragon, BUT not fully.

(S3)This was where a C.D, (Celestial Dragon), casually attempted to purchase a female to make their wife and disregard some of the ones he had. Sanji is the by-product of such an evil practice, his mother was most likely selected by a Celestial Dragon, and through this he was conceived.

Unlike his mother, Sanji would have the blood of a C.D running in his veins, thus in their eyes making up far above ‘’Commoners’’, just look at their reaction when a Donquixote attempts to leave (S4)

So that being said, Sanji is a descendant of these sadistic bastards, So now, let’s go back in time, a Celestial Dragon has grown tired with Sanji’s mother, and decides to return her and keep his son with him, this infuriates Sanji who attacks his father causing him to sentence his Mother to death to teach Sanji a lesson, he would have Sanji executed but because of his half Dragon status decides to lower him to servant status.

Sanji, loses his mother but somehow escapes and disappears on a random ship, fast forward to his discovery by Zeff.

Is it really improbable that it couldn’t be? Surely a child with the blood of the highest Nobility would be immediately recognized in North Blue, if posters were made?
Let’s examine the coincidences in the manga:

1. The first Pirates we meet from North Blue are Bellamy and his crew (S5), Sanji never meets them
2. Sanji chose the following codename on Alabasta Arc : Mr. prince (S6)
3. The only wanted poster which does not reflect what he actually looks like (S7)

This is especially a good coincidence considering it would be near impossible to assume he would be the runaway noble.

Before you argue appearances can change, please note all the children Shichibukai in the following pic (S8). As you can see “distinctive features are often retained”, in Sanji’s case his hair and curly eyebrows could be a dead giveaway.

Sanji also made the following statement to Kuma, during Zoro’s attempted sacrifice (S9). This could be easily construed as his courageous attempt to save Zoro and take the proverbial bullet for the team. However it is a double edged statement where Sanji is hinting at his true lineage. It's the hint that somebody says that "he's well aware of his identity".

It can be all relative coincidences for now, but looking at the Straw Hats themselves, a son of the man who is attempting to bring down the Government, being partnered by a man whose father is part of that very government.