That is not a good way to detect a memory leak. You are looking at the total free memory on the system, which could go up or down due to pretty much anything happening on that box. It only worked for you because the leak was so large.
in reply to Busted: Strange memory leak question. Please help!
in thread Strange memory leak question. Please help!
Far better would be to find the pid of your process and run ps l on it periodically. Look at the VSZ column. If it never changes, then you don't have a leak.