-Can you think of any other possibilities?

Why certainly. This very thing happened to my Mother, while visiting my sister in Tucson, during a trip to the August Farmer's market at Arivaca. She called her sister after being unable to start her 740 wagon in the parking lot, even after being helped by a man in a Ford pickup truck with jumper cables.

The key would turn, and no sound at all would come from under the hood.

Her daughter made the trip South to pick her up and get a tow for the Volvo, but when she arrived, found Mom had left the shifter in "D". So all that was needed was to place the gear selector in the "P" position, which had the effect of completing the "neutral" start lockout switch circuit so the battery current would then find its way along the wire to the starter's solenoid terminal.

Both found their way back to Tucson in air conditioned comfort without the need for a tow.