C++, 16 bytes

main(){new int;}

I don't have valgrind to check it leaks, but pretty sure it should. Otherwise I would try:

main(){[]{new int;}();}

C++, 16 bytes

main(){new int;}

I don't have valgrind to check it leaks, but pretty sure it should. Otherwise I would try:

main(){[]{new int;}();}

Valgrind result

(It does leak indeed)

==708== LEAK SUMMARY:
==708==    definitely lost: 4 bytes in 1 blocks

C++, 16 bytes

main(){new int;}

I don't have valgrind to check it leaks, but pretty sure it should.

C++, 16 bytes

main(){new int;}

I don't have valgrind to check it leaks, but pretty sure it should. Otherwise I would try:

main(){[]{new int;}();}