The difference is that your Sentence 1 expresses a complete thought.
But your Sentence 2 is incomplete (expresses an incomplete thought). For Sentence 2 to be complete, you need to specify some time in the past that is previous to the time that the subject she returned home. For example:
Sentence 3
When she returned home from the office, she was shocked to see that her husband and daughter had been missing since noon or since Tuesday or for an hour or for a month.
All four of these past times are previous to the time the subject she returned home.
Unless you specify some past time, the sentence is incomplete in thought.
And yes, had been missing is past perfect continuous.
When talking about things in the past, the past perfect happens before the simple past.
In your example
If someone feels that they had never made a mistake in their life, then it means they had never tried a new thing in their life.
had never made and had never tried are two actions which were started and ended in the past compared with how the person feels (present). The equivalent would be
If someone feels that they never made a mistake in their life, then it means they had never tried a new thing in their life.
where never made is nearer in time than had never tried since one would have had to try before failing.
I had called you
I had gone to the wash room
could individually use the simple past
I called you
I went to the wash room
with similar meaning, however, using past perfect sets up a scenario for something in the more recent past
I had called you before I went to the wash room
I had gone to the wash room before I called you
To answer where you were when they called:
I had gone to the wash room when you called me
means you were in the wash room (past perfect) when the person called
I went to the wash room when you called me
means you heard the phone ring then went (simple past) to the wash room
Best Answer
a) When we reached the platform, the train was already gone.
b) When we reached the platform, the train had already left.
In other words, the train had already left but we didn't know this until we arrived on the platform. It is the order of events that is key here.
Order of events from both a) and b):
The Train left -> We arrived at platform -> Discovered train had already left