Thanks for assistance and advice, I have checked PIR as suggested and since then no further false alarms, I can only think that it must have been an insect of some kind or perhaps a reflection of some sought. Anyone know how large an insect can set off alarm or if reflections can set off alarm?

Or how small?

I've seen PIR's without sealed optics (or those with but case distorted slightly due to fitting) triggered by storm flies (sometimes called thunder bugs), the size of a pencil dot, when they get inside the unit. Must admit its only systems out in the sticks near farmland though.

Or how small?

I've seen PIR's without sealed optics (or those with but case distorted slightly due to fitting) triggered by storm flies (sometimes called thunder bugs), the size of a pencil dot, when they get inside the unit. Must admit its only systems out in the sticks near farmland though.

You're not kidding! Those little blighters can get in anywhere!

I remember one summer in the early 90s I went to at least 9 calls in one day and very false alarm had been generated by thunder flies.

They play havoc with smoke detectors as well.