The adversary in this case decides to go for a hardware attack, specifically opening the computer/laptop case and removing the RAM to then dump the contents before searching for the password.
So, the only way for this to work would be if the computer / laptop is turned on, and the drive / volume has been decrypted. If you are actually dealing with sensitive information, and need to make sure that nobody can access it other than yourself, do not walk away from a computer that has been decrypted. Shut it off first, then walk away. There is no reason to walk away from your computer if there is a very important volume to you that is currently decrypted. It's like walking away from an open safe that holds 100.000 usd. Close the safe before randomly doing something else.
I also know basic OpSec is to never leave your machine unattended. Lots of other things could happen, user could even melt the screws to make opening the case extremely difficult... but this is not the question! Please focus on this one issue.
But leaving your machine unattended winds up being the core issue nonetheless. You are asking for problems by walking away from a decrypted machine.
If for some reason you really want to have a blow up switch, you would have to make some kind of hardware modification with a simple circuit, ie if laptop computer opened up and certain button is not pressed within 5 seconds set the RAM and drive on fire.
Be nice and play ball with this question. Yes it is a "tinfoil hat" question, but this kind of stuff is interesting for some of us :-). Thank you!
Well, if one has a tinfoil hat on, my question is how did some random attacker manage to gain access to one's computer while he is away in the first place? Shouldn't that person also invest something into physical security? Better locks on doors, fully alarmed house with back up generator, things like that? Worst case scenario is giving someone access to a decrypted computer.
Don't let that happen.
Figure out every possible step it would take to reach the computer, and ensure that there are no mistakes. Don't have a rigged out house? Pay a contractor to rig out your house with the best security possible, claim that your old home was invaded and you don't intend for it to ever happen again if you think that it might bring up weird questions. You could also go the route of claiming you want to add something different to your house to make it stand out on the market. Some people really like having lots of cameras that watch all possible entry points.
Last thing: Do not decrypt anything sensitive in public unless absolutely necessary. Just look at mr. dredd and see how that worked for him.