The April 11, 1864, report in which that remark appears was a sort of "catch-up" roster -- a special report requested by the Trans-Mississippi Department from all regiments in the Department -- which gave the current status of all commissioned officers. On that report, Assistant Quartermaster Dennis Behen, 37th Arkansas Regiment, was reported to have been dropped from the rolls at some point prior to April 11, 1864. No reason was given.
"Dropped from the rolls" meant that a soldier was no longer a member of the regiment. This could be due to death, discharge, resignation, transfer, desertion, etc.