Mission 75 – Steal Santini's Gun Map
If you just finished cleaning out his money cache, you should be right next this location- jog on over, heading in through the switchyard. Drop down into the far side of the roating platform pit, and jimmy a door for a convenient point of access.
Take out the lone sentry in the corridor, then go right, up some shallow stairs. Clear out this area (two goons inside the lounge, one outside), and grab the tacvest.
Use the lounge doorway and near crates to whistle kill nearby baddies, keeping in mind that the nearest enemy may be above you (on the other hand, sometimes your whistle will cause enemies to fall off a ledge and plunge to their deaths, so you've got that going for you).
When only one sentry remains on the upper level, take the stairs up and dispatch him, looting the nearby office
Collectible Alert: Vargas Painting here.
Go back down the stairs you took up, and sneak up behind the last few goons guarding Santini's map. Take the map, blow up the crates, and get gone.