Posted earlier about having no idea what's going on and just did this test, could see a super faint line and now it's darker, not sure if there's colour or not but I've only ever had either a positive or blank white (no matter how long it was left) on these before.
Put a light behind to see if that showed the colour and it looked like the line was see through rather than solid. Anyone else had this? Guessing it's most likely a horrid evap that I've managed to avoid so far 😩