Walter is usually taking a smoke break outside or is inside tinkering with the various machines inside the plant. Speak to him, and you can begin the unmarked quest Treatment. In the back room is Leo Stahl's desk, and you can speak with him about his drug problem or break open the desk.
Assorted chems found in the back of the plant hidden in a (very hard) locked desk.
The "locked desk" remains named "Locked desk" even if the lock is picked.
When the pointer is aimed at an object or person in the plant then moved off that object, the cursor will flicker and show the drink option for a split second, and timed correctly you can drink from the object or non-player character and gain HP and a few rads but note the "drink" text is red and doing so is considered a crime.
In case Walter is missing see his article for solutions.
If the "locked desk" is picked (very hard) and all of the contents are stolen, Leo won't have any inventory at night, even if you haven't started the Leo's Drug Habit quest.