Filters? very vague sorry - please please help

[unhappy]

We moved into our home 4 years ago. We inherited a very nice built in Neff oven and hob - above our hob in the structure of the surrounding walls is a round hole - at the weekend some residue fell from the hole onto the hob - I have prodded inside the hole and its feels like some sort of filter (perhaps) which is obvioulsy in need of replacing - any ideas where or if such a filter exists ie. something that does not go in a traditional hood but in the structure of the walls over the hob - I know this sounds very weird - any advice help desperately needed - thanks