Hi,
is it possible to tell me why your sentence is correct?
"when checking data and documents" is not a complete sentence
True, it's a phrase. Here are some other phrases that use 'when'.
When in Rome, do as the Romans do. (a famous saying)
When in London, I live in a hotel.
Please turn off the machine when finished.
If you like, you can consider them as reduced forms of longer clauses.
Best wishes, Clive