There's the following sentence in my Grammar book:
It was a lovely surprise to find that all the washing-up HAD BEEN DONE while I was asleep.
The only answer mentioned in the answer key is past perfect (in capitals). Since today, I've thought that we use past perfect to clarify the order of actions but if it's clear by itself we don't need one.
I feel, in this sentence the order of actions is obvious, so WAS DONE is also possible. Even more—the past perfect isn't necessary.
Would someone please explain which tense should be used and why?