Ala. Code § 40-7-31
Whenever the tax assessor knows or learns of any property, real or personal, subject to taxation in his county, the owner of which he does not know and which is not embraced in any tax return made to him on or before the third Monday in January by any taxpayer, he shall list and make upon the proper blank a return describing said property according to the best information he can obtain and assess the same "owner unknown" at the amount specified by law on a fair and reasonable market value, and in any notice or advertisement or motion for judgment of sale it shall be described as so returned; and he shall also note the failure of the owner to make such return and shall also note the accrual of the penalty of 10 percent of the taxes to be assessed thereon. No lands shall be returned to "owner unknown" until the assessor shall have made a demand upon the person, if resident in the county, or by certified or registered mail, if nonresident, whose address is known, to whom such lands or property was last assessed, and the said assessor shall make a diligent inquiry to ascertain the name of the owner of said lands or property. Any assessor or deputy assessor who fails to comply with the requirements of this section shall be guilty of a misdemeanor and fined not less than $25 nor more than $500.
Ala. Code § 40-7-31 (1975)