virtualbox.org

I was hoping someone here could shed light on what I am hopping was a false positive. I use virtualbox for basically everything even surfing the internet. Yesterday I was performing a Google search on my Linux image and my windows AVAST! triggered an alert the moment I clicked on a website link within my image, "Malicious site blocked from virtualbox.exe". Maybe I am forgetting something, but I was under the impression the guest and host (unless you configure them to) are not aware of each other's existence? Could someone help me understand how my HIPS antivirus detects my activity on my image? My concern is potential break outs of malicious software.

My host OS is Windows 7 64bit
My virtualbox version is 4.1.18
The image used during the time of the event was Linux Mint
There is no guest additions installed with this image
The network type used for the guest is NAT. (could this be the reason?)

IMHO While the guest and host are not actually aware of each other while using NAT, I assume this trigger was because the guest is actually using the same NIC and a good probability that your anti-virus is in stealth mode it would sample the information passing through said NIC.