In total, about 32,000 BTC were withdrawn from the exchange per day, which is about 19% of all volumes.
Weird that the volume reported by Bitmex has spiked, yesterday was at 165,430 BTC now it's 224.318 BTC, would that all be people liquidating?
Either way, I don't know why somebody would be confident enough to keep funds there, even if they reside outside the US.
Is there any possibility for the American Government to withhold funds?
I doubt it, it's more a case of Bitmex not enforcing AML rules, they haven't accused them of being involved in any money laundering or pointed out cases like that with known customers using them for this purpose, so probably they will not move to seize the servers are accounts, and besides they would have already done it.