Search notes:

/usr/share/keyrings/

/usr/share/keyrings is a directory
man sources.list says that /usr/share/keyrings is the recommended location for keyrings managed by packages (while /etc/apt/keyrings is the recommended location for keyrings managed by the system operator).
At least on my system, the files under /usr/share/keyrings are exclusively *.gpg files.
A file under /usr/share/keyrings might be referenced in a file found in /etc/apt/sources.list.d/.

See also

keyring

Index

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