Go outside and look for close sources of RFI signals, CB/Ham radio aerials, local taxi ranks and police, fire and ambulance stations.
Place the aerial of your mobile phone against the PIR cover and standing perfectly still so you don’t activate it, dial a number and press send. This test can be performed on any power detector.
Mask the troublesome PIR securely with a business card or PVC tape and put the circuit on a soak-test
If RFI is identified as a potential cause of fire alarms, don’t fit a dual microwave/PIR. Microwave is a receiver of RFI and could give more false alarms than an ordinary PIR