A process which doesn't exist cannot hold memory. But the OS can certainly chose to defer the erasure as long as there's no better use for that memory. This is often done to speed up the performance of processes which are frequently quit/stopped and reopened/started.
> A process which doesn't exist cannot hold memory
Not quite. Some leaks are across processes. If your process talk to a local daemon and cause it to hold memory then quitting the client process wont necessarily free it. In a similar way, some application are multi-process and keep some background process active even when you quit to "start faster next time" (of act as spywares). This includes some infamous things like Apple updater that came with iTunes on Windows. It's also possible to cause SHM enabled caches to leak quite easily. Finally, the kernel caches as much as it can (file system content, libraries, etc) in case it is reused. That caching can push "real process memory" into the swap.
So quitting a process does not always restore the total amount of available memory.
