I assume you are using BSPs?
The issue is that part of the mesh is “outside” on a lit area, and the lightmap is “not set up properly” or the lightmap resolution is not sufficient.
If you are using 3D modeling software, using a second UV channel for lightmaps and setting that up properly solves most of these issues. (Similar to texture UV maps, but use lower resolution for them and set them up at the second channel) If you have no idea of what any of this means, it sounds scarier than what it really is. Figuring out the basics of a modeling software helps a lot with designing levels or making interiors, even if you don’t end up making the materials yourself.
With BSPs, you can try to set up meshes in a way where you don’t get situations in which one part of the BSP is in a lit area and another one isn’t, though at the end of the day you are supposed to be using Static Meshes anyway.
I understand about the UV maps you spoke of. I’m using. I will try to reconfigure the lighting and the volume. Thank you ^^.