It's easy!
To keep trunk locked independent of the central locking system:
Turn the master key (key with the red dot) completely to the right and pull it out in that position. Valet key will not unlock it now.
The trunk can now only be opened with the master key by turning it back to the left to vertical to remove key. All the way to the left unlocks locked doors.
So a glance at the key slot shows:
Vertical -- locked if doors locked.
Horizontal-- locked all the time.

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They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety. Benjamin Franklin, A.D. 1759

Roger
1975 240D
1986 560SL