Where did my memory go 'ps' not showing me

Why do you care? Memory is there to be used. Whether it stores zeros, garbage or something potentially useful makes no difference. Ie, what Linux does is to use memory to cache programs it has run. Since people have a tendency to reuse programs they just used, this means that they are already buttembled in memory and do not require reading them and their libraries from disk. If the memory space is needed, by something running, the cached stuff is thrown away and the memory is used. It is ONLY when swap memory starts to be used that you have problems. This means eitehr that your program is using too much memory, or your program has a memory leak.

So, do not worry about the fact that memory is being put to good use. Enjoy it.

Yup, caching old programs.

I do worry about your swap useage. 3GB of total swap? Sheesh that is huge. Why? and 700MB used, that is also huge. That indicates some problem in your program or in some past program ( eg opening 2GB of image files).

Swap should not be used. It is only there for emergency use.

I have no idea which program actually used the swap memory.