Toggle SpoilerIt's in sector 12 (Lucas' laboratory), in the NW corner of the building. However, enemy will blow up the safe containing the journal once they detect you in the sector. The journal is not important in terms of completing the game, but if you really want to get it, you have to be stealthy until you have either retrieved the contents of the safe or disabled the explosion trap. Check the following link for detailed tips on how to do that. I remember having replayed that sector a lot of times before I managed to get the journal intact. Iirc, there was no prize for returning the journal to brenda, and your day will end when you find the journal, so try to get it in the afternoon just before your day would end anyway.