Coolant Reservoir Low

[GUERINO1]

Have a 2007 Explorer 4.0 and filled the coolant reservoir about two weeks ago. (not quite empty) Fast forward 1200 miles. Looked at the reservoir today and it's at the same level it was two weeks ago. (Not quite empty) The oil looks good so I don't think its the head. The radiator level seems fine and cannot see any leaks on the ground. I have noticed a coolant "smell" when I park and walk past the front of the vehicle. There seems to be a light film on the inside the reservoir. Can any coolant be used in this vehicle. There are some coolants that claim they can be mixed with any color coolant.
Thanks.

 

[BrooklynBay]

You could get a cooling system pressure tester from Autozone's tool loaner program to see if there is a leak.

 

[RickMar]

I would say the coolant in your coolant reservoir is replenishing the coolant in your radiator which you are slowly losing over time. Possibly you have a very small leak in your coolant reservoir or radiator.
I would have a leak check done on the radiator just to be safe.
Winter is coming and repairs are much easier completed at this time of year.
Good Luck;
Rick