In my experience those "return" tubes do get wet over a period of time. I would of course check the tightness of the injector fitting but it is more than likely the little return lines.
They are easy to replace, especially using say needle-nose pliers to help pull them off and attach the new line.
While you are doing the leaking one, do them all. They tend to get hard and crack over the years.
A dealer or this site will sell you lengths of hose to do the whole job.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)