Weird thing, lately I noticed that every time I run a command with sudo, it doesn't ask for password.

Thing I've checked:

• /etc/sudoers.d = empty

myuser@MYPC:/etc/sudoers.d# ls
README
myuser@MYPC:/etc/sudoers.d# cat README 
#
# As of Debian version 1.7.2p1-1, the default /etc/sudoers file created on
# installation of the package now includes the directive:
# 
#   #includedir /etc/sudoers.d
# 
# This will cause sudo to read and parse any files in the /etc/sudoers.d 
# directory that do not end in '~' or contain a '.' character.
# 
# Note that there must be at least one file in the sudoers.d directory (this
# one will do), and all files in this directory should be mode 0440.
# 
# Note also, that because sudoers contents can vary widely, no attempt is 
# made to add this directive to existing sudoers files on upgrade.  Feel free
# to add the above directive to the end of your /etc/sudoers file to enable 
# this functionality for existing installations if you wish!
#
# Finally, please note that using the visudo command is the recommended way
# to update sudoers content, since it protects against many failure modes.
# See the man page for visudo for more information.
#

• /etc/sudoers = not even a single NOPASSWD entry

#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"
Defaults        use_pty

# This preserves proxy settings from user environments of root
# equivalent users (group sudo)
#Defaults:%sudo env_keep += "http_proxy https_proxy ftp_proxy all_proxy no_proxy"

# This allows running arbitrary commands, but so does ALL, and it means
# different sudoers have their choice of editor respected.
#Defaults:%sudo env_keep += "EDITOR"

# Completely harmless preservation of a user preference.
#Defaults:%sudo env_keep += "GREP_COLOR"

# While you shouldn't normally run git as root, you need to with etckeeper
#Defaults:%sudo env_keep += "GIT_AUTHOR_* GIT_COMMITTER_*"

# Per-user preferences; root won't have sensible values for them.
#Defaults:%sudo env_keep += "EMAIL DEBEMAIL DEBFULLNAME"

# "sudo scp" or "sudo rsync" should be able to use your SSH agent.
#Defaults:%sudo env_keep += "SSH_AGENT_PID SSH_AUTH_SOCK"

# Ditto for GPG agent
#Defaults:%sudo env_keep += "GPG_AGENT_INFO"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root    ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
#%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "@include" directives:

@includedir /etc/sudoers.d

• groups: not knowing what else to try, I tried to leave my user in the only group sudo, but it didn't change anything, anyway this are the actual groups:

uid=1000(myuser) gid=1000(myuser) groups=1000(myuser),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),104(input),118(lpadmin),128(sambashare),133(libvirt),138(ubridge),143(render),997(docker)

as asked by muru, "sudo -l" output:

myuser@MYPC:~$ sudo -l
Matching Defaults entries for myuser on MYPC:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin, use_pty

User myuser may run the following commands on MYPC:
    (ALL : ALL) ALL