Maiden is an optional quick effect that has to chain directly to whatever is targeting it, as denoted by the "when" text. It is not a trigger effect; it does not trigger. You have to manually activate it.
So, if you activate an equip spell, targeting Maiden, and your opponent responds, you cannot activate Maiden's effect (unless the effect at CL2 is targeting her).
If your opponent targets Maiden, you have the option to chain Maiden's quick effect when you can respond to the activation, as per normal chain rules.
Since both Maiden and negating an activation require direct chaining, only one can occur. It is impossible to have both.
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