@CowperKettle I literally can't count the number of ELISAs I've done in my life - probably somewhere in the mid-thousands if I had to guess. It's just one of those things you pick up on after a while - you accidentally leave a plate out on the bench during development, and it works exactly the same as the plate you ran yesterday!
@CowperKettle most likely. If they're using the standard TMB two-part substrate, I believe one of the components is somewhat light-sensitive, which is one reason why they ship in opaque bottles. However, it would have to be exposed to bright light for a very long time (days to weeks to months) before you'd see any effect in your assays.