I watched Rothko's Rooms last night, a documentary about Rothko's paintings and particularly the seagram murals that I have been trying to see. Yes, they were at the Tate Britain (when it was originally just the Tate) and yes, they were at the Tate Modern. They are not being displayed now, unfortunately. Some of the other murals that were intended for the Seagram building are in Chiba, so I missed out on that too, I guess. Oh well: at least I'm not batshit and the murals were here at one time. Hopefully they will display them again soon.
A late Rothko, getting closer to his death, although his daughter, in the documentary, warned against reading this as somehow indicative of his mental state deteriorating.
UPDATED: FOUND THEM! They're in Liverpool until 21 March.