It’s Impossible to Prove Your Laptop Hasn’t Been Hacked
If you care about this, then put the laptop in a tamper-evident bag. Those are necessarily imperfect too; but there's work making tamper-evident seals to resist up to state-level attacks, since that's relevant in stuff like enforcement of nuclear weapons treaties. That succeeds to the extent that you can find a physical effect that's easy to create and measure, but hard to recreate deterministically. (In concept, dump a pile of glitter over your thing. The effort to dump the glitter, take two pictures, and compare is small. The effort to recreate a given glitter distribution flake by flake is large. Likewise for laser speckle from random rough surfaces, and many other effects.)You could check a laptop for malware later by reading out literally every bit of nonvolatile state, including the BIOS and stuff, and confirming that all changes had expected form (to files you meant to work on, etc.). Of course, then you have to trust the equipment you use for that...A little weird that he ran the experiment. Did he really suspect that malware was routinely getting installed by attackers with physical access to laptops during business travel? If yes, then why didn't someone notice it calling home or whatever?