Meresti security password is a paper note in Fallout 3.
Location
It can be obtained from Vance in the Meresti Metro station, either from his corpse or through dialogue, from Karl using the Black Widow perk or with high Charisma/Scoundrel perk from Holly. With it, you can access the terminal which grants access to Ian West's cell.
Transcript
The password to unlock the security door blocking Ian's holding cell is "Vespertilio".
