No way they looked at all the emails. If there are 650,000 emails and there is 10,080 minutes in a week that means FBI analysts working 24/7 this past week would have reviewed 64.48 emails every minute. There is no way they had the time or the manpower to work on this 24/7. If they used software searching for key words then I imagine a lot of data was missed.
IMHO Comey is betting on HDRC winning and is rolling the dice. Maybe Loretta Lynch showed him some of the rigged EC data for Tuesday the DNC has already released to the press?
They used software to find exact dups of the known classified. Then used software to look for the keywords for classified. They implied they found some new classified. But I agree with you that using keywords would miss some classified especially since the classification headers have been stripped out.