While I agree with another commenter that in all likelihood it isn't running the same software but rather the core is opened sourced and they have proprietary v

While I agree with another commenter that in all likelihood it isn't running the same software but rather the core is opened sourced and they have proprietary value added tools. For example, many OSS SaaS products I've seen have an open source version that only supports running a single user or single company but their deployed SaaS version is multi tenant. So if you're really concerned about SaaS products slipping in back doors or something like that that defeats the "publicly audit-able" condition (if I'm reading your concern right) you can't do anything about it except run the OSS code yourself and not use the SaaS.