To me I believe the security check has been done before issuing MR, though, there is possibility that visa offices operate in different ways, for example applicants in ACCRA VO are receiving MR, and RPRF at the same time.
Ca7r10na said:As far as I understand - 3rd line means results all ok - if they weren't you would instead be contacted by letter. Security screening should start now but it's unclear whether they are in fact doing this beforehand as a few people have got RFPRF and PPR very shortly after third line (yoda within 2 weeks) and the longest I think there has been in recent times with forumites is 6-7 weeks so v hopefully the end is in sight? (But my lawyer's position is that 3-6 months after medical results received is not uncommon for ppr). Good luck!