I don't know what it looks like for a certificated system, but syslog records the private key used for login in a fairly vanilla Debian. If you worry about thi

I don't know what it looks like for a certificated system, but syslog records the private key used for login in a fairly vanilla Debian. If you worry about things like that and aren't looking at physical access (as suggested elsewhere), you presumably have remote syslog and audit which you can check.