Sounds like a leak from one of the hoses going to the oil cooler which is integral with the radiator.
If the level is just below the mark I would start by adding half a pint at a time until it comes to the top mark.
Are you using the correct procedure for checking the level? Engine hot (at least 10 miles), idling in Park for one minute, wipe the stick with a clean lint free cloth (not paper towel) etc?
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)