Sorry I wasn't clear - I agree Environmental trigger shouldn't work. I did have a similar discussion on using Environmental trigger for a fire in the room (Muto Vim and target effects' Frequency modifiers in enchanted devices) - in that case Troy was able to show a canon example of it being done. I reckoned it was a mistake in the example, but things then got sidetracked before any resolution.
I'm not convinced that standard triggers have to be performed by a person holding the item, just that that's the default if you don't make them otherwise. I'd argue the counter to your second point is that light is easier to detect than an aura mundanely. Even for most magical beings, it's easier to tell whether it's light than whether you're in an aura (where you usually have to rely on deduction). We know that magic items have some way of detecting sound (or they wouldn't be triggerable by command words), so them having some way of detecting light doesn't seem that implausible.