This can be achieved once you have access to UNATCO HQ for the first time (After finishing the statue on Liberty Island) When you reach Level 3, find Alex's office (across from Medical) and near his painting will be two floor tiles you can interact with to reveal a hidden crawlspace. There is an assortment of goodies down here icnluding darts, tranq darts, flare darts, soy food and cigarettes.
Here is a screenshot for reference: