Having just completely rebuilt my 123. It looks like the sway bar bushing. I don't see how it would fall of the car. Mine required some manipulating to get them into the uppercontrol arm. The sequence had to be done just right or they would not go in. Have you had the suspension rebuilt at anytime? The mechanic could have dropped it in the engine compartment and it just now got loose.
Look at the front of the swaybar as it comes through the upper control arm. If the bolt came off than the front rubber bushing could come off. The picture below is of the 123 suspension.
The attached picture shows the sway bar going into the UCA. Just to the right of the white shock cover and behind the upper control arm ball joint is where the sway bar attaches. I do not know of this is the setup on your 190. But Mercedes ususally builds things using the same philosophy.
What is the dimension of the piece. Could it have come from the engine compartment?
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car