At the end of acklog.txt I can see the following few lines.

What is CAT? How can I locate the memory addresses that aren't freed by the code?
Thanks in advance.

I guess, you used sys_marker in your code (to localize unfreed memory) or your using of 3D Gamestudio functions is wrong.

Yes, both of them are true

I am struggling with memory issues, I am considering to change the title of the topic.

sys_marker creates unfreeable memory?

No, definitely doesn't.

The engine sets markers itself. I sometimes come across them in acklogs. I think it's when you mess up memory related stuff. CAT could come from a str_cat instruction (str_cat may need to allocate new memory for a longer string).
The issue might be somewhere else though, because as you know memory issues are a pain.

Thanks for everybody for the help. Luckily, I was able to eliminate the issue.
Special thanks for solving the meaning of CAT to Superku.