SIL study of F&G system

Hi everybody here!
I have problem for SIL verification of an exiting f&g (fire & gas) detection system,
the problem is evaluating the amount of PFD (probability of failure on demand).
In my opinion PFD evaluation for f&g system is different from common SIF in process system,
because in the case of exiting a disturbance in process all redundant sensors sense the disturbance and the probability of failure for all of them is equal. because they are a system of for example 2oo3 and we evaluate SIF's PFD for them.
But in analogous f&g system by 2oo3 voting system, there is no equal probability of sensation of gas concentration by three sensors. one or two or three sensor detect the gas cloud and when we evaluate PFD we are not sure that they sense the cloud really or no!???? so we dont know there is any demand or no?! and if we have not any demand the calculation of PFD will be meaningless.
therefore How we can evaluate the amount of SIL?