I recently had my heater core out and the fan/blower unit out. When re-installing the blower unit I became very familiar with all the nuts and bolts that held it in place. That washer may belong to the bolt that attaches the blower box to the transmission tunnel.
At the bottom of the blower unit is an "L" shaped bracket that attaches the unit to the top of the transmission tunnel. That 10mm bolt goes through the bracket and into the transmission tunnel. You attach the washer and 10mm nut to that bolt through the access hole on the right of the transmission tunnel. I would remove the access plate (entails peeling the carpet back), feel up to where that bolt passes through the transmission tunnel and see if there is a washer behind that nut.
If anyone had removed that blower unit in the past they may have gotten impatient and just threaded the nut on the bolt after the washer fell into some unreachable spot. I know because I had to make several retrievals of my washer and nut with a magnetic retrieval tool until I finally got it threaded.
Just a theory.
RWH
