I doff on 10 characters and I have NEVER seen it, do you have to do something in order to even have the chance to see it or what ???
Nothing special really, it's just an ultra rare assignment. As far as I can tell, it only ever shows up on the Medical Department Head assignment list. So far, I've only done the assignment on each of my characters a few times, mostly by bridge invites from friendly people who happened to have the assignment on their Medical DH. Remember this though, you only get a Borg DOff from Reclaim on a Critical Success, which will most likely be Green Quality (with a even rarer chance for a Blue Quality DOff instead), with traits that are more or less the same as the blue and purple Borg DOffs from the STF reputation store. So the Reclaim DOffs aren't exactly 'must haves', unless you're an obsessive completionist.
If you must have them though, just check your Medical DH everywhere that you go.
I've seen it a couple of times, and been invited to bridges a few more times to get it (it shows up in Department Heads - Medical). Overall, it's a pretty crappy mission. You get nothing unless you crit it, and both times I have critted it, I got a green liberated borg doff that's useless to anyone that is not just starting out.
I only encountered it once, on another player's ship, with 'their' Medical department head. It does seem this assignment is a bit 'too' rare, given how much borg stuff we do in this game, already. It was 'supposed' to also show up in old Omega Leonis space, but I stopped bothering to go there, ever since STFs became public ques, so w/e there.
Course one can 'buy' Borg doffs off the reputation system store, OMEGA side, so this 'is' a completionist deal here.