assuming there were no further issues since this was posted? i just had low coolant discovered during an oil change (no warning light but heater wasn't heating up as its getting colder out) so doing similar to you... no leaks found, no smell, oilooks good... topped it up and will monitor but gives me an uneasy feeling
In your situation, there is definitely air in the cooling system that needs to be Burped out.
Just like a a baby needs burped, to get air out of their "lines".
Inspect the ground for any stains or leaks, if none, pull the engine to cover that area.
Also handy to place alarge peice of cardboard down, if area has old stains so you can any new ones.
Take off the radiator cap. Visually insure the fluid levels are full. If not, add fluid.
Get into truck and turn heat to hottest setting and blow out the front vents. roll down your windows so you can stick and in front of the air coming out of those vents to test for Warmth.
Start and run the engine.
When it reaches the temperature of your thermostat, the thermostat will open and push the coolant from your engine bloc into the radiator, forcing the coolant from the radiator, into the engine block. as this happens, you might even see the coolant "flowing" as you look in thru the radiator cap location.
When this is done, walk back to cab, put your hand in front of the air coming out fo the vent and check for warmth.
Bend down and inspect eh ground for leaks .
Check radiator fluid level, once again, after the Exchange has occurred and verify it is full.
if not, add more fluid.
allow the Coolant exhange to occur oen or more times, checking for warmth out of vents and seeing if you are getting more ornot, inspecting ground for fresh leaks and checking flujid levels through the radiator cap location.
Once the engine to radiator coolant exchange has occurred a couple of times and you have no needed to add fluid, replace radiator cap and fill coolant reservoir to proper level.
Like engine oil levels, inspect coolant reservoir levels, regularly.
Any questions, jsut ask.
Someoen will speak up