Yeah, bone fragments is the current understanding on the subject. A lot of birds will also occasionally eat small pebbles and bits of rock to aid in digestion of food... The pebbles help grind up food material in birds' gizzards, then are expelled along with food waste. When condors (and other birds) mistake pieces of junk for pebble or bone, the micro trash often gets stuck in their GI tracts and accumulates over time, especially for nestlings that are smaller and unable to pass the trash effectively.
The thought is that back in the Pleistocene when condors were really numerous - and when there was a lot of megafauna on the landscape like bison, elk, deer, antelope, bears, wolves, etc. - there were a huge amount of big carcasses on the landscape, and lots of bone chips accessible to the birds. So even now condors have an instinctual urge to consume bone chips for calcium, bone development, and food processing.
When performing nest entries, a lot of the trash I've seen does indeed look similar to natural stuff: pieces of plastic, metal, ceramic, and paper products that look quite like bone fragments, mammal hide, etc.
