So far in OB we have been shown that a sphere releasing a "dark light" are used to signify a trapped unmade. We know Galivar gave one sphere to Szeth after their balcony fight [WoK] which he hid somewhere in the mountains of jah kaveed and Eshonai took another from Galivar when he showed her [OB] and told her to show her leaders. Dalinar possesses a sphere containing the thrill at the end of OB as well.
What is interesting to note is in the first Szeth interlude of WoK, and I haven't seen anyone bring this up yet, he is under the command of a man by the name of Took, who and I'm paraphrasing here "was going to tell his friends about the time he stole a sphere that glowed black from the nightwatcher if they would buy him another drink, and Szeth mentions that that story reminds him too much of the sphere he took from Galivar." Which sounds WAY too specific for someone to make up, although took is a drunk and is know to Szeth to be a liar. I think that this specific story, coupled with the fact that the man's name is Took, are too significant to ignore the fact that they're might be another trapped unmade lying somewhere around roshar. Whether or not Took was the one to steal the sphere or not, I think it is more than likely that the nightwatcher had a bound unmade, with a possibility of possessing more.
This brings our total of known spheres up to 3, with a speculative 4th lying around.
I think it is quite possible that Galivar might of even had a 3rd sphere lying around in his castle, perhaps that's what the fuzed are digging for after they captured the city. I'm aware that a popular theory is that they were digging for the truthspren that Hoid took at the epilogue, but AFAIK that is just speculation as well.
Regardlesswe know that almost half the unmade, if not more, are trapped in these mysterious spheres.
Any thoughts/comments on where any other spheres/bound unmade might be? Have I missed any?