It is the nature of bisexuality itself that renders it invisible in plain sight. (The rule of the excluded middle...?)
As the brilliant Marjorie Garber has noted, bisexuality is not so much a category as a category-buster.
Here's why: There are a few people (like the aforementioned Ian the Tall) that actually maintain a menage-a-trios, but the vast majority of bisexuals are working things out in a very different way. The most common pattern among bisexuals is that they maintain a long-term commitment to a stable relationship with one gender, and they have occasional short-term things on the side with the other. In either case, when you are meeting them you are reading them through the guise of a seeming monosexuality.
Let's take for example a married man who occasionally has very short-term flings with other men. Most people who meet him will know him as a married man and assume he is str8. A minority of people will meet him while he is pursuing his interest in male/male sex, but to them (once he has slipped his wedding ring in his pocket) he will look indistinguishable from those who are just plain gay.
(Statistically, by the way, about the same number of bi men actually fit the opposite profile. That is, they maintain long-term gay relationships, but when the moon is full and the partner is on a business trip, will hit the sack with a women for a one-nighter. Same phenomenon, just in the photo-reversed flavor.)
The odds that when you encounter an individual s/he will have no obvious affiliations that would lead you to believe s/he is already either het or homo, and is openly and equally interested in going home with a partner of either sex are very, very low.
Where are they? Well, the highest odds (of course) are that they go to any place where self-defined hets and self-identifying homos both congregate: the opera, clubs, and recreational resorts, etc.