The case of the missing oil ('89 Honda Civic)

This one is weird, trust me. My daughter got her Grandmother's 1989 Honda Civic last spring to have for school. Believe it or not it has under 60,000 miles on it! Obviously Granny didn't drive it much. Everything was fine until August when my daughter drove on the freeway for a couple of hours and the oil light came on. She went into a service station where the mechanic said she was over 2 quarts down. Which was strange because two weeks prior she had a full service oil change. We had our local Honda dealer (Newport, OR) take a look at it since there was no evidence of any oil leaks or any oil smoke at all. He couldn't find any reason other than the person who did the oil change may have lost count of how many quarts he put in. So he filled it up and off she went to visit relatives in Bellevue, Washington, about a 5 hour drive. Two hours into it (around Portland) the light came on again and she had to have a station put oil in it. Then 3 hours later when she got to Bellevue. Bingo it was back on and low and behold the oil was nearly gone! But no signs of a leak or smoke. Off to the Bellevue Honda dealer who took out the plugs, checked for leaks and flushed a very rusty radiator and cooling system. But he too could find no reason where the oil has disappeared to. So, they filled it up again and she returned home to school in Corvallis.A 5 hour drive. She just got into town when the oil light went on again. She put in some oil and that's where we stand right now. The car seems fine around town. The cooling system flush cleared out apparently a large amount of rust (if that means anything). So does anyone have an idea of the missing oil? I'd like to see Agatha Christie solve this one! Thanks for listening! - Jim Bob, Oregon