That's a good guess which I'd considered, but I'm pretty sure that's not the case. This happens even when the laptop is out in the open air, when it should be able to receive a fix on many satellites even if the lid partially obscures it. In that case I'd expect it to reduce the number of satellites (and have its HDOP go up) rather than a sudden 100 percent POOF GONE.

Also, this happens across models.