It depends on how vulnerable to prosecution he is. These days it is hard to completely drop off the earth if you want to live large. The mansion and limo attract attention. It would be easier to live off 200,000 a year in California. If he leaves the US and starts moving around vast quantities of money, he would certainly get busted somehow. Someone will track him down. He knows he has a lot of enemies. He's not that stupid. Evil yes, stupid no.
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tl:dr summary: Miscavige's hidey hole undoubtedly exists today, as do substantial foreign bank accounts. Uncovering the location of his hidey hole before he gets to use it renders it worthless, a worthy goal indeed. That also renders any money he has stashed away essentially worthless, because an infinite amount of money wouldn't enable his personal safety at that point.
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The trick is to track Miscavige's safe haven down now, before he uses it. I am sure he has his escape plan already in place, and has even practiced his own "blow drill" more than once. The money is almost certainly in place already. Some people can be very difficult to dislodge once they have reached their hiding place; I am sure Miscavige's team of expert "wog lawyers" has figured out the perfect place for him.
To track him down now, recent escapees from Int Base might be worthy interview subjects, particularly the person who was in charge of all those real estate deals for the Ideal Orgs (Angie Blankenship, IIRC). There are at least a handful of people who know what Miscavige has set up for himself, and some of them have to have blown recently.
If the veil of secrecy for his hideout is lifted before Miscavige escapes, that place is useless and he then has to start all over to look for another one, a process that could take years, leaving him mightily exposed in the meantime. That will send him into a major rage, which adds some nice lulz to the mix as well.
Regarding the cash: We who work in Global Capitalism HQ are required to take training at least annually on money laundering -- not how to do it, but how to detect it. Our training gives us some understanding of bank secrecy rules and other issues. I thus know for a fact that bank secrecy is not what it used to be. But not all countries have laws that make it easy to tap bank accounts for legal judgments. So I suspect that Miscavige's retirement fund may be somewhat easy to track down and freeze but it might be hard to get a local court to surrender it to settle law suits.
But if the Feds get involved, such as in a criminal case, or in an attempt to collect lots of tax money from rich people, that is a very different matter. The US has been very willing to apply substantial muscle to get countries with no extradition treaty and with abundant financial secrecy laws to make an exception.
Note that the famously secret Swiss banking industry gave up the names of tens of thousands of account holders who were evading US taxes in the last year or two -- a first for them. Any smaller "offshore" banking haven won't last a minute against the financial might of the US government, especially after unplugging Iran from the SWIFT global financial network has seemed to work well, devastating their economy literally overnight.
So if the Feds want Miscavige after the cult has collapsed, they're ultimately going to get him, extradition treaty or no, bank secrecy or no. But they're going to have to want to get him and they're going to have to be willing to put in the effort to pierce through all the layers of corporate shells that he has erected in order to do it. So there's some political will involved. Because we can't count on their willingness to chase after Miscavige, the best way to ensure that he never gets a good night's sleep is to expose the hidey hole before he has to use it.
Any thoughts on who might know something would be welcome.