I have a 2008 s550 and have notice coolant in the air inlet duct on the drivers side

 

I had same issue, there was a hairline crack in the radiator where the air vent attaches.
I bought a new radiator off ebay for around $150.00 and 2 hour install from indy tech

 

This is where my coolant leak was at air intake of radiator

 

thoroughly dry the area when engine is cold.

let the car reach operating temp and let it run for a few minutes more to make sure thermostat is fully open.

observe area of leakage. you should be able to see where it leaks from. for your setup radiator cracks are common in this area

 

I have the same problem on a 2007 S550 can someone tell me a way to fix without replacing radiator

 

Yep been there done that.
While I was there I also replaced the alternator. Charging was a little flakey and it was so easy to get to with the radiator out of the way. 2 birds with one stone...