Have you guys heard of them? There’s a department store near me that has yellow ceiling mount fire alarm horn/strobes.
You sure they were fire alarms? If so, they probably got painted… something that cannot be done.
The plastic could be yellowed from age and sunlight exposure.
Yes, each of them has three red fire labels on them. I don’t see why they’d paint them considering they’re located on ceiling tiles.
Hmmm. The ceiling mount remote strobe in the restroom is also yellow, but the restroom is in the back of the store, has no windows near it, and does not experience sunlight.
Yellowed from lighting occurs as well. Fluorescent lighting does that as well. I have a white Avaya wall phone I just replaced in my basement it began to get a yellow tint from being under the lights for so long.
I’ve seen that once, at a Value Village in my city. This Spectralert that was once white is now yellow for those reasons.
No backbox, how nice 
I guess it is probably from lighting. There’s a horn/strobe in the area with black ceiling tiles that’s still white. It was probably able to resist yellowing due to getting less light.