"After an opponent plays a location, place the card on the bottom of the deck to choose that player..."

Can this card be played on an already placed location or basically only if you have the card in hand when your opponent plays a location?*

I mean, if it's 'any opponent location' why doesn't it just say 'choose an opponent location' without seemingly attaching some sort prerequisite?

*Ex: Starks have three locations in play. Lannisters have initiative and in the draw phase, pull a Shield Islands Droman card. Can the Lannisters play the card and then activate the response to target one of the Stark locations? Or must they wait until the Starks play a fourth location down the line?

1. You can only use a response just after whatever you are responding to happens.

Putting that together, to use Shield Island Dromon (a location), it must already be in play itself, then you must use the Response immediately after an opponent's location is played (ie, it can't be used on locations that are already in play when it is played).