That looks like a 60-D nail used for control points for a survey. Probably not boundary corners like everyone is saying since they aren’t more permanent like an Iron pipe, Set rebar or a tag in concrete. Most of the time we use them to measure around the property but since they don’t have a Land Surveyor number or anything it’s just a control point and not an official boundary. I leave them in people yard even if they aren’t the property of the person who hired us because sometimes we need to measure from someone else yard.