@tthbaltazar @mjg59 It's not trivial to sniff if you use a fTPM or similar, since then it's buried deep inside the CPU itself. But yes, physical access can still be a risk, since the attacker could for instance sniff the memory bus; but that's a more complex attack than "plug a USB stick and boot from it" or even "sniff the low speed LPC bus".
(Some computers have "intrusion switches" to sense when the case is opened, but I don't think they set any flag the TPM can use as a sign to stop.)