I've had evaps or strange looking faint line indents showing up on every single FRER I've taken for the last few months. I'm really starting to hate these tests. Hopefully, yours is a positive, but I have had some strange lines as recent as last week show up IN the time limit with FRER. I took the tests apart because the line was questionable and it's just a faint white-ish looking indent. So disappointing.
I'd retest with another brand or wait a couple days and see if it shows up darker.