Thank you for your sage advice!
The sliding door seems to be involved with this self-activation of the door locks; at least the behaviour changes quite noticeably after the treatment you suggest.
After (forceful) unlocking/locking of the sliding door, the self-activation, while not gone altogether, was greatly improved (it happened far less often).
Should I take the door apart to 'see' what's going on? Or is the problem even in the door?
AGAIN, thanks for the wise advice!