Those 30% are not lost coins, but 'zombie' coins.

NVIDIA engineer John Ratcliff calculated in June 2014 that “zombie coins”, defined as those which have lay dormant for at least a year and a half, accounted for 30 per cent of all Bitcoins.
Those coins were bought for just a few cents, so the lack of profit-taking despite a return of 4,000 per cent seems to rule out the possibility that they're simply long-term savings.

For more detailed analysis of 'disappeared' Bitcoins (lost coins, zombie coins, satoshi's coins, burnt coins etc.), read the full article here:
http://www.telegraph.co.uk/technology/news/11362827/The-625m-lost-forever-the-phenomenon-of-disappearing-Bitcoins.html