What does the tilde at the end of a file name stand for?

If a file is appended with a tilde~, it only means that it is a backup created by a text editor or similar program; it does not suggest another program is writing to that file. Besides, any file opened in vi cannot be opened in another application, but that is another discussion.

To easily find all these type of files, type cd to get to the top level of your home folder and enter:

find -type f -iname '*~'

Or, if you want to find them in a specific directory, type:

find /home/mike/Downloads -type f -iname '*~'

These files can be very useful if you happened to have edited a file and have not saved a backup of the original! Gedit, for example, creates such a backup of any file edited, although you can turn off this setting in the preferences. However, the (files~) are no substitute for a proper backup, as they might not contain the data you need, or be the version of the edit you wanted.

Some text editors (such as emacs) create a backup of the file you are editing, and give the backup file the name of the original file plus a tilde. The file you describe might be the result of opening scan.log with such a text editor and making some change.

You can use fuser (see Ubuntu package psmisc) to see if a process currently has that file open: fuser scan.log~