My understanding, and this is confirmed in one of Qorax's own posts are that there are 2 parts to the "Background checks":
Part 1 - Document checks, which happen when you go "In Process", before medical requests are issued. This is being convinced of authenticity of work experience documents, and contacting employers if necessary, and also confirming authenticity of proof of funding, educational documents, birth certificates etc (they use third party companies to perform some of these checks).
Part 2 - Security checks, which happen AFTER medical results are received - and involve background security checks via a number of different agencies.
So when Qorax is saying that background checks are ongoing after you receive medical requests, I think he means Part 2?
Wayne.