@Vovachan
I've never come across any engine that had fire warning systems inside the core. How hot do you think it gets in there?
On the RB211's I have worked on the fire warning loops are positioned on the outside of the fan case, the outside of the core, in the nose cowl, in the inter-services area( the splitter) and in the strut. I see no reason for the Trent 900 to be any different.