I would like to know if we could write "What had happened while we were away." I think past perfect is not obligatory in time clause.
When we returned from our holidays, we found our house in a mess. What had happened while we had been away? A burglar had broken into the house and had stolen a lot of our things The burglar got in through the kitchen window. He had no difficulty in forcing it open. Then he went into the living-room.
Just would like to know if we could write "What had happened while we were away" . I think past perfect is not obligatory in time clause.
You are correct, "were away" will do. Apart from being understood, there are some nuances.
The story is about what they found when they returned from vacation, that is, what had happened while they were away. They found that a burglar had been there and had stolen some of their possessions..
You might want to continue the past perfect: the burglar had gotten in through the window. He had had no difficulty forcing it open. But that's not necessary, although if you shift to simple past: the burglar got in.... you're no longer looking back in time at what you discovered when you returned, you're telling a slightly different story, how the forced entry took place.
Historical present can be substituted for the past to give greater immediacy to the story. The past tense can be substituted for the past perfect to the same end.
When a single event in the past happens, simple past is fine.
When I went into the room, the vase was broken. What happened to it.
In your example:
What had happened while we had been away?
the burglary happened over a period of time in the past. They had to enter, search, take things and leave. The additional text confirms this:
A burglar had broken into the house and had stolen a lot of our things The burglar got in through the kitchen window. He had no difficulty in forcing it open. Then he went into the living-room.
In this case past perfect is better.
Though in reality it is a fine distinction. Either would be understood correctly.
