They get brittle with age, then they crack and fall out. Not a big deal if it's not there, one of mine was missing for several months before I noticed. Best to have that hole covered though, prevents road debris flying up in there and fouling the ring gear.

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles