ID and OD at flush joint connection are almost same as pipe body unlike coupling connection.
Is CCL able to detect very small magnetic anomaly?
In other words, doesn’t a pup joint of flush joint work as a depth maker?
Have you ever successfully take CCL in flush joint CSG?
In my experience it CAN be difficult to pick up clear CCL in flush-joint casings. It can depend on how much a CCL signal is processed or digitised by the contractor's system.
As a marker, I would recommend adding a radioactive pip-tag to either the top or bottom connection of your pup joint.