IDM without SLO: redirect SAML logout to warning page


I use SAML with a Shibboleth server, which doesn’t provide SLO (single logout); “IDP SLO Redirect URL” isn’t set. Rocket.Chat Account setting “Forget User Session on Window Close” is set to “false”, because with set to “true” Apps on Android and IOS lose connection quite often. If an user logs out from RC the IDM token still exists and if the user doesn’t close the browser, one can login again without any credentials. As a workaround after logout I’d like to redirect the user not the the RC start page, but the another page with a warning hint to close the browser to finally logout. Where would I change that in code?