Is there any app or log that says when my MacBook Pro's hard drive last went into sleep mode? I ask because each night when I close the MBP's lid without being plugged into a power source, the power light goes into its snooze rhythm but the battery is half-drained by the next morning. It certainly looks like it is asleep, but the power drain seems to indicate otherwise.
I put my macbook to sleep and checked system.log file. Then I used mdtuil to reindex spotlight (which prevents the hard dive from sleeping while its indexing) then I put it to sleep again. I compared the log entries from both sleeps and there was no difference. So it looks like this can't be done using OS X logs.
Read through this Apple support page Mac OS X: Why your Mac might not sleep or stay in sleep mode and see if anything there helps you.