Unmarked

an unmarked highway patrol vehicle

My mother died when I could not have been more than eight or nine years old, on one of old master's farms in Tuckahoe, in the neighborhood of Hillsborough. Her grave is, as the grave of the dead at sea, unmarked, and without stone or stake.

The “little green men”—faces covered, wearing unmarked olive uniforms, speaking Russian and using Russian weapons—have played a significant role in both the occupation of Crimea and the civil war in eastern Ukraine.¹⁹⁶

Bale worked Bozovic again as he was left unmarked in the Montenegro area but could not get enough power on his header from a looping cross.

The teacher sat down to a pile of unmarked work.

The use of that word in that context was unmarked.

For instance, from the semantic point of view the words dog and bitch are unmarked and marked for the contrast of sex. The word dog is semantically unmarked (or neutral), since it can be applied to either males or females (That's a lovely dog you've got there: is it a he or a she?).