True, but they sorta backed themselves into a corner this season by revealing the exact numbers of the Seven.
It just suddenly occurred to me that if the Final Five are actually exploring the Cycle of Time, then it would make sense that, if Three thinks the Fifth is among the Cylons, that, in light of Douglas' comments, the Four must have gone to the Colonies and the Fifth, like the 13th tribe went elsewhere - namely it found the Cylons that had left the Colonies.
From this, I speculated that may well be the key to Cylon evolution. If the Four were exploring what it means to be human by going to the Colonies, maybe the Fifth decided to explore what it means to be Cylon by living among the Cylons.
Since the 13th tribe evolved into the inhabitants of Earth (Cylons of the "Five Five" type - hereafter F5 type), then it seems to me to make sense that in the process of exploring what it means to be human, it somehow, someway participated in Cylon evolution. It may be what they think of as their "God" - not just the abstract God (the One Who Cannot Be Named), but maybe the Cylons somehow got the idea that it (the Final Cylon) is God. Since God is said to have made man in His own image, then maybe it made Cylon skinjobs in its image (or the Cylons began making themselves into its image, either way could work). Then, the Cylons, against the will of the one we call the Final Cylon, decided, instead of being friends with humanity, to destroy humanity. Consequently, that's why the Final Cylon is said in the Prime Hybrid's prophecy to be "in the darkness, seeking redemption that will only come in the howl of suffering." It (unwittingly) set off the series of events that led to the destruction of humanity (like Baltar on the Colonies) in its efforts to understand what it means to be Cylon. It is therefore hiding somewhere, either among the Cylons (which seems to me to be rather odd if true) or using a new face - maybe taking the body of what would have been Number 7 and leaving the Cylons to hide among the Colonists, or being the actual Number 1 with the other numbers being changed by Cavil to hide the truth. Therefore, that could account for why Three thinks it is among the Cylons - it was
at one time, but it has either taken a new face (one that doesn't look like the one in her visions) or its face is unknown (having been wiped from Cylon perception after Cavil's little "revolution," so it is actually among the Colonists or elsewhere altogether (if a physical entity) - unknown to anybody, and not hearing the music, since it never "switched off" its identity, which was the reason the music was needed to "reactivate" the Four.
One of the things that makes me think that Cavil isn't telling the whole truth and that the Final Cylon is the real Number 1 is his name. Just as Gina's last name meant "resurrection," Cavil's name has a meaning too. The word means to quibble, to offer petty objections. Why would a Cylon take such a name for itself, and why would it be Number 1? It makes sense, at least to me, that it may mean that it "caviled" with the real Number 1, and consequently, just as it got its way against the Twos, Sixes, and Eights in S4, it may well have gone against its wishes and tried to get rid of it, which would account for his animosity toward the whole concept of finding the Final Five. Maybe he's met one - the one we call the Final Cylon - and he caviled with it and won the argument, maybe by staging his own revolution or keeping it isolated from the others somehow, maybe in a virtual prison (boxing it). It got out, and, like the Twos, Sixes, and Eights, left the Cylons and joined with the Colonists, unbeknownst to any of the others, and probably before they came online. Cavil then took the Number one for itself, got rid of the body, and the others came online. Somehow the Final Cylon escaped and was downloaded into body 7 and then, before more copies could be made, escaped and hid among the Colonists. After the attack, it blamed itself for what happened, and has been seeking redemption ever since.