in night raid report no. 348 (11./12.6.1943), targets Dusseldorf, Duisburg etc. are mentioned. The plan of attack of Dusseldorf includes the information that

En route to the target, yellow warning T.I's were to be dropped by Oboe Mosquitos [at specific coordinates]. The marking was to be maintained by Lancaster backers-up, aiming yellow T.I's at existing yellows.

I wonder why these warning T.I's were dropped (during other raids too). What was the warning for? Did they not show the German nightfighters the route?

Can anybody explain this to me?