bibtex-files is a variable defined in `bibtex.el'.
Its value is

List of BibTeX files that are searched for entry keys.
List elements can be absolute file names or file names relative to the
directories specified in `bibtex-file-path'. If an element is a directory,
check all BibTeX files in this directory. If an element is the symbol
`bibtex-file-path', check all BibTeX files in `bibtex-file-path'.
See also `bibtex-search-entry-globally'.

You can customize this variable.