If they open for trading but there's suspicion they're no longer fully solvent, withdrawing BTC will be the fastest exit. BitstampBTC may therefore trade higher than other market's BTC IOUs.
What if the hacker sells the 18k BTC on Bitstamp? (if they're stolen)
Then the folks at Bitstamp will thank their good fortunes, seize the funds and sic the cops on the hacker.