Antifreeze missing????

[Tmacmaster23]

Hey everyone...this is my first post on this site hoping to get a little info about an issue im having with my 2001 Explorer Sport....it has the 4.0 6cyl with about 126k on it.

The issue is that in the antifreeze backup tank i had filled it to the mark that says "cold full "...then a couple days later it was gone.
I then looked under the oil cap for moisture and dip stick for color of the oil and everything seems to be normal.

I have no smoke of any kind out the tailpipe and no smell of the antifreeze and the truck warms up like normal.
the heater in the truck seems to blow ok hot air but not as hot as i thought it should be. So i then checked for leaks in radiator and hoses.

Im so confused at this point im not sure what to do.
Could it be a bad thermostat?
and its just going through the antifreeze faster cause its stuck open?

thanks everyone and hope to hear from someone who can help me.

 

[swshawaii]

Rent a coolant system pressure test kit that's fully refundable when you return it.
Almost guaranteed to find your leak, without guessing. Most auto parts stores have them.
Call around. GL