I have now become convinced that what I saw was an evaporation line. I've never seen ANY sort of line on a test before, so seeing the barest hint of a line this morning made quite an impression on me, but I've also never used this brand before. So, I did a little research and it turns out the blue dye tests are notorious for showing evap lines. Apparently, the thing to look for is color, and if the line you see is at all blue, it's positive, but if it looks like a shadow, then it is most likely not positive. What I saw was a shadow. Like I said, it just looked like I could see where the line would be if there had been a line. The picture below is NOT mine, but the brand of test I took looks similar to this one (the line I got was much MUCH fainter than in the picture below):
So at this point I am going to just assume it was negative, but if anyone has more experience with HPTs than me (I've never been much of a POASer), please let me know what you think.