unknown flash memory in valid address

Check FLASH_PCROPxxxR and FLASH_SFR.

What should I need to check for exactly?

Check the contents of those registers and interpret these values according to the description in the RM.

The few last sectors of the main flash memory are protected against any direct access as it is the secure part where the Firmware Update Services (FUS code) is loaded. This code is the only one able to load the RF stack in a secure way.

Any sector above the one pointed by the SFSA option byte is not accessible.

When the RF stack is uploaded, the SFSA pointer is updated accordingly, preventing any access to this stack.

Even the Cube Programmer tool cannot access this part of the memory. When you try, it only returns 0.

​Yes. As yo can see from sector 0xCB (should be the calue of the SFSA option byte),it is nor possible to acess sectors above where the RF stack has been loaded.

This protection is effective as soon as the chip is leaving the manufactory as it is pre-loeded with the FUS code.

Thank you very much