security mounting question

If I am adding a smoke detector to my hardwired security system along with a new siren like a system horn only, would I need an old work back box like the plastic ones or do I not even need one because I know for low voltage applications like CATV and DATA you don’t need one, and the code stated that any device that has brackets for mounting that is low voltage does not need one. just want to check?

sorry wrong area, reposted in security help

In the future, just ask one of the mods to move the topic and we can do that for you.