Although I may be mistaken I think NFPA 72 requires fire alarm junction boxes to be 'identified'.
Spray painting covers red is cheaper and faster than labels that say "Fire"
Yes, in chapter 9.7 dealing with Public Cable Plant
"Cabling systems and interconnections
between alarm transmission equipment and alarm-receiving equipment shall comply with the requirements of Section 9.7." It appears to be aimed more at a municipal system than anything else.
I couldn't find it anywhere else in NFPA 72. There is this from NFPA 70 760.10:
"Fire alarm circuits shall be identified at terminal and junction locations in a manner that will prevent unintentional interference with the signaling circuit during testing and servicing."
Note that it doesn't specify the manner of identification. A tag inside the JB might be sufficient.