Can't remember which exact engines anymore, but some are prone to get an air pocket when filling with water. After driving awhile due to heat/cold cycles & water in motion as thermostat opens & closes the block can burp itself & then fill up to level, lowering radiator level. This could be what you're seeing.

On all engines I now drill a hole about 1/8" or 3mm in diameter in the flat plate of thermostat. This acts as a bleeder hole when filling radiator, giving captured air an escape route. You then fill, wait a couple of minutes, refill, wait etc. until it won't take any more water in radiator.

I've had more than my share of problems with leaky heater cores. It can go undetected if a very slow leak because it's often released as vapor. A hint of this is an unexplained fogging of the windshield from the defroster ducts. Over time it can rust out the heater box too! Some models of that vintage Dodge heaters were built in such a way as to be able to trap a small amount of water without leaking right out visibly, can't recall which heater model # that was.

I'd also double check all radiator & heater hoses......