If Nick doesn’t mind - I’ll chime in too, since my house used to have those sort of smoke detectors.
When you press the test button on the detector, does the FOS horn go off? Or has the horn ever gone off when the smoke detector is accidentally set off due to things like cooking smoke/burned food?
Also, I’m not sure if I’d recommend a blowdryer to dislodge the dust from the smoke alarm. A vaccuum cleaner usually tends to work better than a hairdryer.