This is what happens after the Visa Office recieved your file:
Once the visa office begins processing your file they will commence with document background checks.
At this stage your ECAS status should now read “In Process” with the extra line in the detail “Your application was reviewed and we started processing on <Date>”.
This document background check will include ascertaining the authenticity of your documents, and since they use some third party companies for this it could take a varying level of time depending on your visa office, the complexity of your case, and even the speed of the individual visa officer looking at your case. They may also perform employment verification checks which will involve telephoning both you (whilst at work) and your employer to ensure you give the same answers on their questions about your employment. Only a small percentage of applicants are likely to receive employment verification. Only if they are unconvinced by any of the work experience documents you sent, or if your application is one of the random spotchecks they do to prevent fraud, will they actually perform the verification.
Visa office may request further documentation from you during the processing phase, which can come by email or snail mail. They may also ask for a few things (PCCs or fresh forms) at the same time as medicals