Metrics That Cannot Be Fooled ??

I would like to initiate a discussion on what are the metrics that can helps us to detect that actual no of defects in the software development process.I would describe the problem here as below :

Most of the Project Managers take a call and decide whether to enter the defects reported from customer or not beacuse more the number of defects , the PM may loose their target .So , to avoid that they do not enter the defects in the system.Especially in case of service & maintenance of projects , they would not take the defects saying this defect is not under our scope and we do not own that module or sub-system.So there are lot of human factor gets factored on deciding on the number of defects available in the system and this lead to have the value the metrics that the PM wants .The PM are aware how much to enter and when to enter the defects .

If a developer during his unit testing gets a defects he fixes the defect before the final release and since it is detected by developer and fixed by himself never gets into the system as long as the it is done by independent system testing team

Therfore the question here

1.How shoould we make the internal system strong so that they are not able to manipulate and the system caputres the correct and actaul number of defects (can we bring some kind of PoKA -YOKE)

2.What are the metrics that can helps us in knowing that some lekage of defects is there( either not entered or not identified ).Are there metrics that cannot be fooled .

3.Are there any measurement statistics which can helps us in preventing such a practice which is so human factored ?