Unmarked (adjective)

1. Not having a mark, label, or sign to indicate its status or identity.
2. Not having a noticeable feature or quality.
3. Not having been noticed or observed.

Origin:

Early 16th century; un- + marked.

Examples:

1. The unmarked car followed the suspect.
2. The unmarked package was delivered to the wrong address.
3. The unmarked trail led to the river.
4. The unmarked envelope contained a letter from the company.
5. The unmarked door led to a secret room.