What are these 2 rectangular rubber covers?

[ML320Truck]

I was under my truck tonight and saw 2 rectangular rubber covers in front of the tranny pan. They are about 1’’ by 2 ‘’ with a small hole in the middle. I pulled one cover off and saw a part number A163 271 0080. They cover a housing between the engine and the transmission. I peeked into the opening and saw a big gear; I guess that’s the starter plate?

The question is that I got a big oily path around the rectangular openings; It doesn’t look like a leak, since the openings are not sealed and inside is completely dry. It more looks like some sort of oil vapor accumulated there with dirt.

Any insights? Should I seal these 2 holes with masking tape