Very faint/barely there pregnancy line on FRER

Hi ladies- this morning I tested and I swear I thought I saw a super faint line, that was barely there. I compared it with one I took a few days ago and on the older one I couldn't find anything that was like that (i was think maybe i was seeing an indent?) 
My question is, has anyone ever thought they saw the faintest line ever and it turned out to be positive? AF was due 2 days ago, but I would've thought that FRER would've caught it.