Just wanted to say I have the secret flap as well! - mine is a Bosch Exxcel WTE84307/GB.
Didn't have problems with container lights etc (yet) Was having problems with a squeal (drum support rollers I think -given then a clean and squeak seem to be cured) -anyway looking around for more info I found this thread so checked my dryer while I was fiddling with it - mine was full of fluff and water too so gave it a good clean out...
I have this same model of tumble dryer and I can't seem to find the secret flap! Can you please tell me where it is? I have got down on the floor and opened the lower left panel, taken out the air filter, cleaned it, removed any gunk in the space it came from and removed a ton of really gross gunk right out the very back and up above as far as my hand would reach! But can't seem to find the flap! any ideas?