it's definitely a light map problem. what i suggest to fix it, is on your second UV channel make your whole mesh a pelt unwrap so it is only one large piece(this will avoid pixel bleeding like the video alex linked above), then scale the edges to the very borders of the UV coordinates. Stretching is okay on lightmap UVs so…
A couple factors to this problem, I think? The main problem, the one I want corrected, is that I am getting shadows cast where my tiling UV shells meet. So the vertical shadows along the concrete border of what hopefully reads as a median are where the UV shells meet. I've never seen this happen before and I'm not sure how…