Crash, Reboot, Comes Up with Keychain for Login Open?

Curious:

On my iMac 5K ("iMac17,1"), running 10.13.6 (yeah, I know, little old):

• I popped it out of sleep with a keypress,
• It asked for my password, so I typed it in,
• Halfway through typing it, it locked the screen again,
• Repeating that exercise, it appears to have been completely ignoring keyboard and trackpad (other than to wake it up, apparently),
• I held the power button down to reboot, and
• It came back up with Keychain Access open for login (not my account).
• There was also a dialog asking for a ... master password (or something to that effect; I can't recall exactly what it called the password it was asking for), but I'm not sure whether it was Keychain Access presenting that dialog; it may have been Seamonkey asking for a password; not sure.

Not sure what to make of that... Anybody seen that sort of thing before? Should I re-lock the keychain for login?

FWIW, I did a Malwarebytes scan of the machine, and it found nothing fishy.