Computer was acting odd when I got home

I was out for a few hours this afternoon. When I returned the CapsLock LED was on but the shift state was off; pressing CapsLock turned its LED off but CapsLock on. Alt and Ctrl didn't work: I couldn't change virtual terminals. Attempts to su failed in either Caps state (the password was rejected?). I pressed the power button, which Linux handled, turning off gracefully (elogind-daemon records this in /var/log/messages). A capture of an audio stream that crond had started 5 minutes before I pressed the power button had worked. What happened? There's nothing amiss in the logs.

Toshiba Satellite C855, about 10 years old.