Joe, I wouldn't recommend using a pre-spray and rinsing with a colloidal. Either a fresh water rinse or better still an acidic rinse will work much better.
Colloids like Mpower require a dwell time to work efficiently and work better as a pre-spray rather than tank detergent.
Most post clean smells, as has been said already, are either caused by the carpet being still damp the next day which allows invisible moulds and mildews to form in the base of the carpet. These offgas but will disapear once all the moisture has gone. The smell is usually like a foisty smell but a bad one can smell like cats urine.
In these instances it is not recommended to re-wet the carpet for obvious reasons.
Some houses these days have very high internal humidity. Double glazing, showering, cooking, drying clothes on radiators all contribute to this. This also prevents the carpet drying out quickly.
Or the other cause is re-constituting some kind of contamination sych as urine, vomit or drink spillage. Only way to determine this is to give it a really good sniff close to the pile. Or as already mentioned, pull the carpet back and look for any staining on the back.