HVAC blower comes on by itself. Even after thermostat has been disconnected from wall. EDIT- POSSIBLY FIXED
Final cause is a combination of 2 problems.


After review of the wiring diagram, there appears NO way power can get to the blower motor except via the 24 volt transformer voltage, which only goes thru my thermostat. There is no other way to get power to the blower. Thus there has to be a short in the thermostat wiring between the furnace and my thermostat. 


Tracing the wiring - I find that there is a midline splice in the line, that is used to tie in the A/C compressor to the circuit. This splice was just taped with electrical tape to the furnace metal gas line outside the furnace.  I found frayed wiring and bare copper showing- thus it appears that that the wiring is occasionally shorting- either from contact to the metal gas line or directly between the wires (why just some of the time I don't know).  Correcting these shorts, the blower now does not come on by itself anymore (no more 'ghost' fan starts), only operating with the thermostat in place or if I short the wires.  However,--


The thermostat itself is also broken.  Possibly due to the shorting that has been going on? It operates the A/C Compressor or heat calls properly based on cycling when the tempature calls for it, but the fan now continually runs, even with the thermostat switched off- unless I remove the thermostat from the wall (prior to fixing shorts the blower would still continue to run, even with thermostat removed from wall).


Thus, all works well except the thermostat seems to have internal shorts that keep the blower fan on at all times regardless of heat or cooling calls, or even when the thermostat is switched to 'off'.  

Replaced the thermostat, and now system works fine.  So I was trying to solve two problems at the same time- always makes for more fun.

Thanks to comments from 'ThreePhaseEel' for pointing me to the wiring diagram so I could isolate problem, and to 'Ken' for telling me where to find the wiring diagram!.