Image: UbisoftOfficial screenshot from Ubisoft.
Hidden Cash unlocks when you find a stash of hidden Moneda in any Special Operation. Moneda is the currency for Lola's black market, so the stash pays off beyond the trophy.
1. Talk to Lola and start any Special Operation. 2. Search away from the main path for the hidden stash. 3. Pick up the Moneda and the trophy pops.
Look for the stash before you grab the PG-240X, since the canister only heats up once you carry it into the sun. Play on Standard difficulty so you can explore without much pressure.