Graffiti #10 is in the burned old mill. Pick up the knife from the bar / wooden beam, turn around and go to the sheet metal and interact (move). After that interact (destroy) with the tag behind the metal and you get graffiti #10.
For a visual guide open the spoiler, inside you find my video with all 10 graffiti locations.
*** Spoiler - click to reveal ***00:08 - Optional Graffiti #1 (American Graffiti)
00:43 - Optional Graffiti #2 (Extra Credit)
01:16 - Optional Graffiti #3 (Peer Review)
02:05 - Optional Graffiti #4 (Monthly Masterpiece)
02:53 - Optional Graffiti #5 (Spit Take)
04:38 - Optional Graffiti #7 (Hello Nurse)
05:26 - Optional Graffiti #6 (Venting Machine)
06:53 - Optional Graffiti #8 (A Penned Appendage)
07:57 - Optional Graffiti #9 (Drunk Drawer)
08:41 - Optional Graffiti #10 (Messed Up)