You start off selling maybe 100 bitcoins which you've 'created', so the value of bitcoins on your exchange is 100 higher than in your 'wallet'. No-one can audit that and no one will notice because it's a tiny amount compared to the total volume. The more you do this, the more popular your exchange looks and the more you can repeat it and get away with it.
Eventually you be holding only a tiny fraction of the exchange book in actual bitcoin having cashed out 90% of it generating large amounts of money for yourself in the process.
If it ever looks like there's a run and you can't provide people with their bitcoin you claim "hack".
By the time you exit scam and claim "hack" the missing coins are gone but really they didn't exist so there's nothing to trace.