Cardiolab will call right away if they see something life threatening. In terms of accuracy it records everything.
Test accuracy will be limited only by the skill of the lab and diligence of the physician.
Yes, the lab would have contacted you immediately if they saw something that was potentially dangerous. After a cardiac event, my lab called back, and demanded a manual recording to ensure I was in normal rhythm. They are contracted to monitor you, and so are consequently responsible for your well-being.