Macbook lists all WLAN-networks of another machine. How and why was they shared?
All stored wifi are stored in the user and system keychains. You can open up Keychain Access and search for all known / saved networks in the system and your user account. 

Repeat for other user accounts and you'll know how or when they arrived. 

iCloud Keychain syncing seems the most logical source along with not changing the default "remember networks I have joined" but rather than speculate, you can go to the source keychains.