reftex-default-bibliography

reftex-default-bibliography is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
List of BibTeX database files which should be used if none are specified.
When `reftex-citation' is called from a document which has neither a
`\bibliography{..}' statement nor a `thebibliography' environment,
RefTeX will scan these files instead. Intended for using `reftex-citation'
in non-LaTeX files. The files will be searched along the BIBINPUTS or TEXBIB
path.

You can customize this variable.