interior read dome light

ok here is one i have not seen until today in my 2008 220 cdi.
the rear interior dome light cluster, for some reason part of it seemed to have dropped down - the centre strip part facing the front of the car, had a look and it looked like a hidden part - nothing mentioned in the manual, so left if, and then when i got home, the part has disappeared, so the cluster looks normal again.
thoughts ?


