reftex-bibpath-environment-variables

reftex-bibpath-environment-variables is a variable defined in `reftex-vars.el'.
Its value is

("BIBINPUTS" "TEXBIB")

Documentation:
List of specifications how to retrieve search path for .bib database files.
Several entries are possible.
- If an element is the name of an environment variable, its content is used.
- If an element starts with an exclamation mark, it is used as a command
to retrieve the path. A typical command with the kpathsearch library would
be `!kpsewhich -show-path=.bib'.
- Otherwise the element itself is interpreted as a path.
Multiple directories can be separated by the system dependent `path-separator'.
Directories ending in `//' or `!!' will be expanded recursively.
See also `reftex-use-external-file-finders'.

You can customize this variable.