Problem with an old SMB connection on High Sierra

Then it has to be a software looking for the connection. If it is not in the automount settings for the OS as it was already removed from there, there must be another software which is trying to access it.

I have seen other posts with similar issues, and the issue was solved checking /Library/LaunchDaemons for programs trying to open that connection.

That only means that the process looking for that server is still running, whatever it is, regardless of the user. The fact that happens on another Mac means that Mac was also connected to that volume.

In Terminal you can try

netstat -nv | grep ip.address.here

If you get any output, the second rightmost number is the process id (PID) of the process attempting to connect.

Also

lsof -i@ip.address.here

will tell you the PID of the process connecting to that IP address.

Then

lsof -p PID | awk '$4 == "txt" { print $9 }'

will tell you, among other things, which files are using that connection.

For example, doing

lsof -i@23.205.189.191

Which is the IP address for this forum, tells me that there is a process using WebKit, meaning a browser.

Is not as straight forward as getting the name of the app, but if you get any output it can help a lot to see what is causing that call to that IP address.

See if this User Tip from etresoft can help Prevent a volume from mounting at startup

As expected, just that there is a process trying to mount a volume from that IP address.

Were you using that server to authenticate users or as storage only? If it was to authenticate, the server could still remain set up in the Directory Utility.

If it was only for storage, I'm afraid I cannot be of further help. I cannot see a way of "guessing" which software is requesting that connection, or in which config file that IP address is still set. From a library still stored in the iTunes config file to a more hidden system process, to a backup software (including Time Machine) which is still configured to use that server, even if not visible in the GUI to an automount script on a cron job.

Perhaps grep'ing the /etc and ~/Library directories is not that bad idea or search if there is an smb.conf file or similar, at this point.

Does it happen to any other user in your computer or only to this one?

Do you have any volume in the System Preferences > Network > Login items or other software than Time Machine which could be running and looking for that volume (another backup software, for example)?

Contact Apple Support at https://getsupport.apple.com

Do you remember which software was using that IP address to connect to? Authentication, backup, storage, that could give a hint on where to look next.