“Used Coffee Department” is a jocular term for a restroom (or, in particular, a toilet).
       
“Decided I’d best go find some repellant, visit the used coffee department, and wake my youngest son to take part in history!” was posted on The Astronomer’s Home on June 8, 2004. “Cold turkey MUST happen when you can’t even count how many times you must leave the room to go to the used coffee department” was posted on Long Island Exchange on April 24, 2006. “Just heard some old guy say he was going to the used coffee department as he entered the bathroom. #truth” was posted on Twitter by Alex Brown on May 23, 2011.
   
“Used Beer Department” is a similar and more popular term. “used beer department  noun a toilet US, 1995 Modified to ‘used coffee department’ and the like for office settings” was printed in the book Vice Slang (2007) by Tom Dalzell and ‎Terry Victor.
     
   
The Astronomer’s Home     
June 8, 2004 - Venus Transit!!
Comments: I was up very early, and enjoying my morning mug while watching live webcam images with Joe and waiting on dawn. The clock ticks… OK. I’m ready. Out on the edge of the east field at a little after 6:00. Both the 4.5 Celestron and the little Orion 4.5 had honours, mainly because the only two solar filters I possess fit them. On hand is the little TV/VCR combo, the video imaging eyepiece, a 35mm camera loaded with good film that can be digitalized, and my camcorder. Taking no chances, I also brought an eclipse viewer with me. Well, by then I had 37 mosquito bites and enough coffee to cruise at 20,000 to Thailand. Decided I’d best go find some repellant, visit the used coffee department, and wake my youngest son to take part in history!
     
Long Island Exchange
Rupert Everett, Early Morning Jitters, And The World’s Biggest Drug Dealer
APRIL 24, 2006 BY JOE WALLACE
(...)
In a hurry to go to the bathroom, that is. After all, the side effects to being a 12-hour-a-day java junkie is an endless run back and forth. Horrible. Cold turkey MUST happen when you can’t even count how many times you must leave the room to go to the used coffee department.
 
Google Books
Vice Slang
By Tom Dalzell and ‎Terry Victor
New York, NY: Routledge
2007
Pg. 194:
used beer department  noun
a toilet US, 1995 Modified to ‘used coffee department’ and the like for office settings.
       
13 July 2009, McClatchy-Tribune Business News (Washington, DC), “Woman finds strength through being generous: And by giving back, she and others hope to honor loved ones, meet needs” by Edie Ross:
“I like to do what I can,” Robbins said. “In the porta-potties in Greensburg, we put up a sign that said, ‘This used-coffee department is funded in part by a passed-through gift from Dick Robbins to the Community Foundation.’ So it served two purposes—it gave us a place to go, and it let more people know about what the community foundation does.”
   
Universe Today
Tammy Plotner
OCTOBER 21, 2009 AT 6:24 AM
(...)
while i was out, i also made a couple of other observations as well:
(...)
. old german shepherd observing partners will steal your snuggie while you visit the used coffee department.
 
Twitter
Alex Brown
@AlextehBrown
Just heard some old guy say he was going to the used coffee department as he entered the bathroom. #truth
3:50 PM · May 23, 2011
 
Twitter
joe felix
@jfelix_the_cat
Potty time. Used coffee dept (@ Rest Area) http://4sq.com/17digJM
1:17 PM · Apr 3, 2013 from Texas, USA
     
Twitter
joe felix
@jfelix_the_cat
Potty time. Visiting the used coffee department. (@ I 10) http://4sq.com/12gDROs
9:45 AM · Apr 24, 2013 from Guadalupe East, TX
 
Twitter
Mal (unwell to begin with)
@TheHudsonBay
This guy just referred to the restroom as the “used coffee department.” #cringe
10:21 AM · Oct 25, 2017
 
Twitter
Andrea Feller
@AndreaFeller2
Replying to @ShreyaTrivediMD
I’ve been trying to find an appropriate reason to share what I heard the other day. Instead of “where’s your bathroom/washroom”, “where’s your used coffee department?”  Maybe an awkward fit in this thread but the whole poop and coffee thing seemed….... ok never mind
4:51 PM · Oct 19, 2018