Elkins joined the Lions earlier this year as an undrafted free agent out of Oklahoma State. The Lions also added undrafted free agent Jon Morgan at linebacker, and he is still on the team as the start of the preseason nears.
It's not clear who the Lions are making room for right now. Perhaps they have decided to sign Rocky McIntosh, Demorrio Williams or one of the other linebackers who worked out for them last week. It's also possible, as Justin Rogers tweeted, that the Lions are clearing room to sign a defensive end or cornerback given the injury issues they've had at those positions.
The Lions return to practice on Tuesday morning, so I'd imagine they will have signed someone by then.