you'd be surprised how much memory content can be found on a powered down machine.
I remember the old sun U10 sparc boxes. If you had one with en elite3D graphcal card in it, and powered down the machine when it was still displaying the Xsun / X-windows system (say when you had a powerfail), then when powering the machine back on the first thing you would see would be the contents of the framebuffer at the time the machine was powered off.
Tried this once before moving a machine to another office
Machine was powered off for a weekend, plugged back into a different office, and the first thing we saw when powering on the machine was the screen as it was when powered down .
It didnt last long, because the sun openboot prom screen dump would start to write and would erase the previous screen.
My point ?
If you really are worried and paranoid, just powering down your machine everytime the doorbell rings wont be good enough.
I would suggest doing what they did in cryptonomicon, and have a big electromagnet built into your doorposts (ok, maybe not practically possible: