So, I took an hpt (ic-threshold of 25) this afternoon with about 2 hour hold (10dpo). Here it is undoctored, very faint there, let me know if you can see it:
Here it is inverted
Honestly, I am thinking it is just a fluke of the test. Within the read time, the area around the line was a faint pink, but never dark, but defined on lines. The darker line appeared after all the extra color junk left the windo. Does anyone else see it?