فَحَصَ الْمُهَنْدِسُ الْجِسْرَ بِدِقَّةٍ.
The engineer inspected the bridge carefully.
Bi-diqqatin supplies a useful detail about how the inspection was carried out. Where the speaker wants to convey not just that an inspection occurred but also that it was thorough, the phrase can have a completion (tatmīm) function. No preexisting suspicion of negligence is required.
Compare the earlier 'quickly, without neglecting safety', which blocked an unwanted inference. Here the more salient purpose is adding a useful manner detail. Context and the kind of benefit are needed for a precise classification.