These fruit had already fallen, but the fruit is sometimes being eaten on the tree. Just one or two fruit at a time. No noticeable teethmarks, though I will examine the intact lemons more often to see the start of the damage
Location is Melbourne, Australia. Variety is Improved Meyer Lemon.
Suspect some invertebrate, though it could be possum or rodents.
I visit this tree regularly, so can do experiments with it.
....and another one in the last 5 days. Found (again) on the ground, open surface down. The stem looks chewed through.