It’s not a stupid question, it can be confusing.
In 2006, legislation was introduced requiring that lots in older strata instal smoke alarms and keep them in good working order. In summary, if you have installed a smoke alarm in your lot then you are responsible for it. If the smoke alarm was installed at the time the building was constructed, then the owners corporation is responsible for repair and maintenance of it, except for changing the batteries in it, which is the lot owner’s responsibility.
So the question for you in relation to your four properties is whether the smoke alarms were put in at the time of construction or whether they were put in by the then-owner. The strata manager of each should be able to tell you, as if the OC is responsible they should have a maintenance program and checks for them as part of their Annual Fire Safety requirements, whereby you get a certificate every year. This certificate has to be displayed on the notice board. When those checks are done a notice is sent out to all owners/residents advising if access to lots is required for the purposes of the inspection/checks – your property manager would get that if there was one.
In relation to the alarm that is hard-wired, I would guess that would be the OC’s responsibility?
You are paying $100 a year each for someone to check if the fire alarms are working and to change a battery (presuming they are your responsibility – it is your call as to whether that is too much or not. However, if you are paying property managers, you could expect them to provide you with advice that is pretty standard for their job, especially if they are recommending the fire compliance service (you have to wonder why).
The Department of Planning put out a Fact Sheet setting out all the requirements and how they work.