When changing smoke detector batteries on my First Alert SA511 smoke detectors, I noticed that ONE (of the six) has a discolored battery tray/door. It appears that the plastic used to make the battery tray was of a different composition from the rest of the shell, as it has turned yellow with age, while the rest of the smoke detector has not. (Also, my other five smoke detectors all have pure white battery trays that match the body as well…)
The detector is mounted indoors on the ceiling with no direct sun exposure so I’m a bit surprised at this amount of yellowing. The tray appears to be structurally strong and the unit tests fine, but I’m going to keep an eye on it in case the plastic starts to fail, which might result in the batteries not being held against the power contacts.
The only difference I could find is that my other smoke detectors were manufactured in 2017 or 2020, while this particular one was a replacement unit that they shipped me for a failed smoke detector and it was manufactured in 2018 (April 11).
The front of the unit looks like this: