Yeah, but sex clean up tissues is not a good example as you state there is likely to be some blood in there and the fact they alerted is unsurprising imo. If the dogs also alert to body fluids such as saliva, semen etc from a living person then we are in different territory altogether. In fact it would be hard to say what the purpose of the dogs is at all. As far I've been able to ascertain the dog only alerts to cadaver odour plus blood.
Much of what i've been able to find out is from a different case and a Q&A from a cadaver dog handler and supposed expert on another forum. She stated explicitly that a dog that is trained to just detect cadaver odour and not blood like Eddie will only alert to that scent and can quite easily discriminate between blood from a dead person and that from a living person simply because of the decomposition. It bugs me that I have to rely on the internet to find this stuff out, but I have found it very hard to acquire any official information about this like a reference book for instance... In the case of the McCanns we know no-one died in the apartment for instance prior to Madeleine's disappearance, but do we know if any of the furniture is second hand and may have been in contact with a dead person? Anyway, these alerts can be misleading due to factors like this and transference of the scent which is supposed to be relatively easy. Add into this the possibility of false alerts and that's why forensic corroboration is vital.
Anyway, regarding the methane point. Perhaps this is a factor that can confuse the dog or mask the smell. However, in a bathroom there would have to be somebody with a serious stomach upset I would assume to give off sufficient methane quanities 
There doesn't seem to be a consensus on what such dogs react to, their nomenclature, training parameters, desensitisation, etc., and the science and training methods change all the time.
How many professional dog handlers are likely to admit that their dogs screwed up?
On a different point, there is no way of knowing whether there was innocent contamination or not. Grime and Harrison did point this out, but the initial PJ team didn't seem to have investigated alternative explanations beyond informal questions as to whether anyone else was known to have
died there.
I don't place the blame
entirely at Amaral & co.,'s doorstep. It's not clear from the files whether Grime had fully informed the initial team verbally concerning caveats once they actually met.
