Crossword Genius

Discovering an unspecified number escaped from imprisonment, put a hundred back in (9)

Ross

I believe the answer is:

detection

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'back in' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Another definition for detection that I've seen is " Discovery".)

I've seen this clue in the Sydney Morning Herald.
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