I think I've discovered a (convoluted) way to hunt down someone who is constantly under the effect of a Mind Blank effect.
It would take a while to get up and running, and wouldn't work against someone who is hidden away and never makes a public appearance. It would eventually work if you at least knew where the target tends to hang out (if they have a base of operations that is near a town, etc). I'm writing this from the view point of trying to hunt down PC's.
First, get a humanoid to start asking around about the PC's. Probably easiest to dominate a low-level character to that effect.
Second, either the PC's hear about someone asking questions, and hunt down and kill the low-level human (perhaps he is dominated and has been ordered to make threatening remarks, but not attack the PC's), or the low-level human actually finds them, and approaches them in a way that sets the PC's spidey-senses tingling. With a little luck, and carelessness on the part of the targets, they end up murdering the low-level human. The fact he is low level increases the chances that they will accidentally kill him outright before realizing how squishy he is.
Third, the target is raised as a Revenant. The monster entry in Monsters of Faerun doesn't give a proscribed way as to how this happens (aside from spontaneously when someone is murdered), but in the adventure City of the Spider Queen is a minor artifact, the Claw of the Revnancer, which has the ability to turn any corpse into a Revenant under the control of the claw's wielder. (I know, it takes an artifact; I told you it was convoluted).
Revenants have the following ability:
Find the Guilty (Ex): So long as a revenant and its killer are on the same plane of existence, the revenant will know which direction its killer can be found in and how far away he is. Depending on the magical abilities of the base creature, this extraordinary sense may even work across planar boundaries.
They are also immune to rebuking or turning.
So far as I can tell, this works even if the murderer is under the effects of Mind Blank (just as Scent or Blindsight would work against someone with Mind Blank).
The Claw of the Revnancer does specify that if a Revenant is ever ordered to seek out its murderer, the claw's control over the Revenant is immediately and permanently ended.
However, nothing seems to prevent Control Undead or Command Undead from affecting a Revenant.
So, trick the PC's into murdering someone, retrieve his corpse, turn him into a Revenant, use Command or Control Undead upon it (another reason why a low-level character is best for this), then tell it to seek out its killer, followed by asking it where its killer is located.
So long as you and the Revenant are on the same plane as the PC's, you can always find them (well, until the Revenant completely rots away, which its monster entry says takes about six months; maybe gentle repose would help in that regard).
