They originally intended the mic to go behind that grill.
However, it turned out that resulted in too much driving / wind noise for the roadsters.
That is why they relocated the mic to the spot on top of the steering column, which - apart from indicator noises - is much quieter. They did so without adjusting the ceiling lights however, thus leaving a “mystery grill”.
If you watch videos for installing after market mics, you will notice some of these still refer to that grill as the place for your aftermarket mic to go behind (as, despite the poorer sound quality, you then can install such aftermarket mic without sourcing a replacement steering column cover, or cutting into the existing one).