Missing oil?

I'm posting on behalf of my father. My father has been flying a friend's
older 172. The 172 was recently flown on a 8-hour round trip. During this
trip, the plan burned 3-quarts of oil. On return, a compression check was
performed. All came back with 75's or better. A check for
leaks did discover two *very minor* leaks but nothing which would account
for 3-quarts in 8-hours.

The engine bay was not covered with oil and no pools of oil were found.
There is not a large streak of oil on the bottom on the plane as one would
expect if it were simply leaking.

All this makes us think that the plane really is burning the oil but
wouldn't the compression check yeild much lower results if it were burning
that amount oil? Where's the likely suspect here?

Thanks in advance,

Greg