Based on the report produced by the National Police Improvement Agency into the disappearance of Shannon Matthews which stated:
"The properties that the dog searched contained a high level of second-hand furniture bought from dwellings where someone had died. This resulted in numerous indications that required further investigation to confirm whether they were connected to the investigation, or to previous owners of the furniture."
The dogs are clearly able to scent odours from items that have been moved around and are nothing to do with soc as above. In this case, MM, we can't possibly know the history of the apartment, its contents and the hire car. Therefore its quite possible the alerts were not connected to the investigation but something similar to above. In the Shannon Matthews case the British police were clearly able to identify the issue ie second-hand furniture bought from dwellings where someone had died, but in the case of MM I can't see where these sorts of issues have even been considered let alone identified and eliminated?
In terms of the human cellular material thought to be blood as detected by the dog, but invisible to the naked eye, we have no idea if a) it was definitely blood b) if it was blood, or some other human cellular material, how did it get there and who did it originate from.
These tests/results are utterly meaningless.