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".