High - creeping memory usage W7 Host Arch Guest

[MarmotteNZ]

Dear all.
For the past two or three version of Vbox, I have noticed that whenever I leave my guest OS ON for several hours, and regardless of it being idle or doing intensive thing (ripping videos etc), the memory usage in the host would creep up.
Last night, I left the guest idle overnight, in the morning the memory allocated in Windows was at the top. My Host was barely responsive.
What is weird is that, neither the processes (including "show processes from all users") or the "Resource Monitor" show anything significant for the VirtualBox process. (it showed then only 30ish Megs used by the rpocess). Nevertheless, shutting down my guest instantly dropped my memory usage, as per the attached screenshot.

I don't know what to do. I don't think I am allocating too much to the guest (see log attached).
It is weird that windows does not even report properly the memory usage in the process list.
Should I suspect a bug in WIndows itself rather than VirtualBox?

[mpack]

And why do you think that VirtualBox is allocating this memory, and not (say) the host data cache doing what it's supposed to?

Does the host OS fail to surrender this memory when asked?

(Tip: we have had this conversation many times. We know that what you're seeing is normal).

[MarmotteNZ]

"And why do you think that VirtualBox is allocating this memory, and not (say) the host data cache doing what it's supposed to?"
I don't know indeed

"Does the host OS fail to surrender this memory when asked?

(Tip: we have had this conversation many times. We know that what you're seeing is normal)."

I don't know/understand the inner working of Windows in regards to memory management.
What I know is that, as per my initial writing, "host was barely responsive" [...until I close the guest machine...]. So indeed NO I don't think it is normal for the host to become non responsive whenever a virtual machine - with no more than half the memory allocated to it - is left running for a few hours) and YES I suspect the host is somehow failing to surrender (or whatever other term is relevant) some memory.

Even if I don't have the theoretical background to understand/explain how this is happening, my practical experience - repeated every time - shows me that the responsiveness of the host comes back when I shut down the guest machine.

I am also experiencing many crash/freeze that forcibly pause the guest machine - or sometimes crash it completely. See log attached..it just happened: