What (type of) sensor can detect odor of faeces?
According to this paper in 2000 (could have changed in 15 years!) there were no commercial sensors on the market specially built for this.  However, the following were tested:

Figaro sensor type    most sensitive to: 
TGS2600               general air contaminants 
TGS2610               general hydrocarbons                         
TGS2611               methane natural gas 
TGS2620               alcohol and organic solvent vapors 
TGS825                hydrogen sulfide                           
TGS826                ammonia


Overall, though, they concluded: high sensitivity to humidity proved so vexing that the practical prospects for this approach were deemed discouraging. An alternative approach using the differential response of a matched pairs of sensors, with one of the pair equipped with a filter that traps fecal component gases and vapors, is now under investigation

I would think though that in 15 years time the sensor capabilities/stability/immunity to humidity may have improved.  So you could check the Figaro website for sensors detecting the same chemicals as the paper and maybe get better results.

Instead of checking all of these individually, you might be able to just check for general VOC's.

Another slightly hokey report on baby diapers

And a patent for a fecal incontinence system