Was it a blue dye test? A few months back I POAS (clear blue) with an expiry date of 2006 and almost immediately got a very strong positive. Rushed out and spent a fortune on pink dye tests only for every single one to be negative.
Fx for you that your in-date test gives you the result you want
Sounds promising, but you won't trust it until you do an 'in date' test. I used a cbd yesterday that was bb 08/12 and it told me 'Not pregnant 2-3' . (this morning's one was in date and much more reassuring!!)
A line is a line. You dont get one without pregnancy hormones! I think being out of date would just decrease the efficacy of the test. It still shouldnt pick up what isnt there IYSWIM! Congratulations!