1. simon closes btc24 -> tell a good story
2. takes our bitcoins and sells them on other exchanges while the price is falling
3. waiting some days/weeks
4. buy back all those BTC for cheaper money
5. open btc24 again with all funds -> no harm to the users
6. profit!
This kind of operation would be easily traceable on blockchain.info - the address(es) of cold wallet(s) are not exactly secret.