@danhash Evernote populates SourceURL attribute when it is empty and user pastes text from the clipboard with one of the clipboard formats containing url of the page the text was copied from. This typically happens when you copy text from the browser and paste it into a note, or drag and drop. Most of the time it's very convenient to have a reference back to the original page -- you have your local copy stored in Evernote in case online page goes away, and also a reference to the source material, if you need to revisit it. In fact, this is very popular feature: quite a few Evernote notes are created with this automatic source url, so it must be useful.
There is no code that populates SourceURL field based on the note content, so the "randomly" populated note SourceURL you see come from you copy/pasting text into your note.
If you don't want clipboard SourceURL to be associated with the note every time you paste web page text, click "Info" button and add a few spaces into note URL. Blank SourceURL will not be shown in the Note Info panel and non empty Source URL will prevent new SourceURL added to the note when you paste.