The answer is in the protocol you are connecting to:
The location of a Sitemap file determines the set of URLs that can be
included in this site map. A Sitemap file located at
http://example.com/catalog/sitemap.xml can include any URLs starting
with http://example.com/catalog/ but can not include starting URLs
There is not a single canonical location, because depending on the location of the file, it can only describe the content "below". So from this definition,
/sitemap.xml is the default canonical path for a sitemap that describes URLs "anywhere" on the website.
Note that this relative uncertainty exists because it all started before the IETF created a specific standard for defining where to place the "well known" files in some web file systems. See well known Uniform Resource Identifiers