$ cat test.c
    void main (void) {
      malloc (1000);
    }
    
    $ make test
    cc     test.c   -o test
    
    $ valgrind --leak-check=full --show-leak-kinds=all -s ./test
    Memcheck, a memory error detector
    Command: ./test
    
    HEAP SUMMARY:
        in use at exit: 1,000 bytes in 1 blocks
      total heap usage: 1 allocs, 0 frees, 1,000 bytes allocated
    
    1,000 bytes in 1 blocks are still reachable in loss record 1 of 1
       at 0x483877F: malloc (vg_replace_malloc.c:307)
       by 0x109142: main (in test.c:2)
    
    LEAK SUMMARY:
       definitely lost: 0 bytes in 0 blocks
       indirectly lost: 0 bytes in 0 blocks
         possibly lost: 0 bytes in 0 blocks
       still reachable: 1,000 bytes in 1 blocks
            suppressed: 0 bytes in 0 blocks

> vg_replace_malloc.c:307

What do you think that is? Valgrind tracks allocations by providing other implementations for malloc/free/... .