Funny coincidence, just came across a site just like you describe last week. Previously I have only come across the usual over specified L5/M instead of an L5 with the smoke management interfaced in but superfluous sounders and call points were included.
However this site had a completely self contained smoke management system with it's own control panel, smokes and manual overrides, yet also had a completely separate fire detection & warning system (MCP and smokes) that only covered the circulation areas, wasn't monitored and serves no apparent purpose.
It's been causing problems due to constant vandalism and the fire service are getting brassed off as (unusually) some residents keep dialling 999 when it goes off.
As the structure satisfies stay put, the client has been advised they can (within reason) do anything to resolve this up to full removal as long as residents are fully appraised as to changes in cover and procedure. The fire service agree that they aren't bothered about it being there either.