my son just purchased a 2002 c240, Im not sure what exactly goes here, but looks like its missing

in front of red cap,

 

so i found a owners guide online for my sons c240, looks like this is the + lead connection to jumpstarting this car.

 

Yes it is the positive attachment point for jump starts. The negative is on the strut tower hump. Look for the thick cable attached to a nut.

The red thing in the photo is actually the cover for the rectangular hole. Looks like the previous owner popped the cover upwards then closed the hood on it. You might want to fish that thing out. I can see that it's damaged.

A new cover can be picked up at the dealer for a few bucks. Hopefully the 4 black "hinge" tabs aren't damaged, though it unfortunately looks like they are.