Do you have any sudo call from your /etc or /etc/skel bashrc or profile...?
Yes, I do have in /etc/bashrc: sudo -l
Unless you already understood: su - "make the shell a login shell" so sudo -l in bashrc is executed, which asks for the user's password
Understood, who is asking - not understood why "sudo -l" is asking for the password and why just hitting the enter key works
sudo -l lists the commands that you are allowed to run with sudo
That is clear to me, but why does this command request the password?
Do you have rootpw/runaspw/targetpw set in /etc/sudoers?