Started wondering over the weekend. Did an Amazon cheapy Monday morning which appeared to be negative, then as I went to throw it I thought I saw something.
Did another after school run, so not FMU. Was the faintest of faint lines (think tilt and squint
).
Did another Tuesday morning and there was another faint line within ten minutes but still not obvious.
Stopped at shop on way back from school run bought different cheapies that I used with ds. Was a pinky line within limit but still really faint.
This morning I did the other new cheapie and still a hint of a line. Realised the Amazon ones are more sensitive so did one of those an hour later and a line was there within time. But you've guessed it...very faint 
I couldn't have five evap lines/false positives over three days. Could I?