Is it possible to pick up serious conditions that might cause fatal arrhythmia (long qt etc) in a routine ecg/holter even if an episode is not caught?

It's possible: It is possible to pick up a long QT syndrome, without any episodes of passing out or rhythm problems. Generally, you'd expect a QTc over 500 ms, before changes in treatment; however, you should see a heart rhythm specialist, or cardiologist, if QTc is unexpectedly prolonged. Medications, gender, and heart rate can also affect your QT interval. Holter measurements may be less reliable than ECG.