def getPassword = { username ->
    def creds = com.cloudbees.plugins.credentials.CredentialsProvider.lookupCredentials(
        com.cloudbees.plugins.credentials.common.StandardUsernamePasswordCredentials.class,
        jenkins.model.Jenkins.instance
    )

    def c = creds.findResult { it.username == username ? it : null }

    if ( c ) {
        println "found credential ${c.id} for username ${c.username}"

        def credentials_store = jenkins.model.Jenkins.instance.getExtensionList(
            'com.cloudbees.plugins.credentials.SystemCredentialsProvider'
            )[0].getStore()

         println "result: " + credentials_store
    } else {
      println "could not find credential for ${username}"
    }
}

getPassword("XYZ")

result: com.cloudbees.plugins.credentials.SystemCredentialsProvider$StoreImpl@1639eab2