This is more of a “sysadmin” answer, but you can use strace to give you stacktraces showing where a given syscall is invoked. Use the -k command line option for that. So if you, for example, try this:

$ strace -etrace=clone -k -y -f -f  bash -c "ls /dev/null;command echo"
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7e1b55ccaa10) = 141358
 > /usr/lib/x86_64-linux-gnu/libc.so.6(_Fork+0x27) [0xee1a7]
 > /usr/lib/x86_64-linux-gnu/libc.so.6(__libc_fork+0x52) [0xf3fb2]
 > /usr/bin/bash(make_child+0x19e) [0x6909e]
 > /usr/bin/bash(adjust_shell_level+0x315) [0x56935]
 > /usr/bin/bash(adjust_shell_level+0x1bda) [0x581fa]
 > /usr/bin/bash(execute_command_internal+0xb88) [0x4ae08]
 > /usr/bin/bash(execute_command+0xce) [0x4dc6e]
 > /usr/bin/bash(execute_command_internal+0x2ff2) [0x4d272]
 > /usr/bin/bash(parse_and_execute+0x7ab) [0xb5ffb]
 > /usr/bin/bash(_rl_enable_paren_matching+0xb0ce) [0x11e2de]
 > /usr/bin/bash(main+0xf78) [0x33568]
 > /usr/lib/x86_64-linux-gnu/libc.so.6(__libc_init_first+0x8a) [0x2a1ca]
 > /usr/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x8b) [0x2a28b]
 > /usr/bin/bash(_start+0x25) [0x34385]
strace: Process 141358 attached
/dev/null
...

(you’ll see the SIGCHLD after since strace will show signals by default, but that part isn’t relevant for the answer)

With your application (tracing only clone(), following children -f) this should tell you where in libgomp‘s code the thread is created.

I could reproduce the behavior:

#include <omp.h>
#include <stdio.h>

int main() {

#pragma omp parallel num_threads(3)
  printf("Hello from Thread %i of %in", omp_get_thread_num(),
         omp_get_num_threads());

  printf("nDone with first teamnn");
#pragma omp parallel
  printf("Hello from Thread %i of %in", omp_get_thread_num(),
         omp_get_num_threads());

  printf("nDone with second teamnn");
#pragma omp parallel num_threads(3)
  printf("Hello from Thread %i of %in", omp_get_thread_num(),
         omp_get_num_threads());

  printf("nDone with third teamnn");
#pragma omp parallel
  printf("Hello from Thread %i of %in", omp_get_thread_num(),
         omp_get_num_threads());

  printf("nDone with forth teamnn");
#pragma omp parallel num_threads(3)
  printf("Hello from Thread %i of %in", omp_get_thread_num(),
         omp_get_num_threads());
}

$ OMP_NUM_THREADS=2 strace -etrace=clone ./a.out
clone(child_stack=0x154b3d1ebf30, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[235919], tls=0x154b3d1ec700, child_tidptr=0x154b3d1ec9d0) = 235919
clone(child_stack=0x154b3cfeaf30, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[235920], tls=0x154b3cfeb700, child_tidptr=0x154b3cfeb9d0) = 235920
Hello from Thread 0 of 3
Hello from Thread 2 of 3
Hello from Thread 1 of 3

Done with first team

Hello from Thread 1 of 2
Hello from Thread 0 of 2

Done with second team

clone(child_stack=0x154b3cfeaf30, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[235921], tls=0x154b3cfeb700, child_tidptr=0x154b3cfeb9d0) = 235921
Hello from Thread 1 of 3
Hello from Thread 2 of 3
Hello from Thread 0 of 3

Done with third team

Hello from Thread 1 of 2
Hello from Thread 0 of 2

Done with forth team

clone(child_stack=0x154b3cfeaf30, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[235922], tls=0x154b3cfeb700, child_tidptr=0x154b3cfeb9d0) = 235922
Hello from Thread 1 of 3
Hello from Thread 2 of 3
Hello from Thread 0 of 3
+++ exited with 0 +++

Whenever the team size shrinks, libgomp seems to drop the extra thread and needs to create a new thread for the next team.