Anything is possible.
If it was a heater core,you should be experiencing the inside windshield getting fogged up and a slimy substance on it as well as the carpeting getting wet/Get under the dash and lift back the carpet near the firewall and look for any signs of coolant and wetness.(Passenger side)
Get this system pressure tested and look for any leaks.
Water pump could be seeping out of the weep hole and you may not be noticing it.
you can use a small mirror to look under the water pump for signs of leaks.Look for any traces of stains near the water pump or down the side of the block or lower hose.it could be minor enough that it dries before you see it.
Rent a presssure tester at autozone.
It could be an internal problem,something thats very minor and not manifesting itself thru smoke that you can see.
If it holds pressure,leave the pressure tester connected and pumped to system pressure,usually like 15 0r 16 lbs and periodically come back and recheck it,it if slowly goes down ,keep pumping it back up to system pressure.let it do this for a few hours if need be or overnight.Then you can remove the spark plugs and look for any evidence of coolant in the cylinders.