I disagree that the price is the last traded price. Currencies are traded in a decentralised fashion over the counter: Dealers of one bank are connected directly with dealers of another bank, primer brokerage services connect 3rd party customers to a dealer, etc. There is just no globally known and accepted last trade. There won't be a publically known tradebook either (which you could average over), parties settle their trades directly.
The price you see when you ask for the current price is the midpoint of the current spot bid and spot offer, taken of Google's market place of choice. That's also the reason why rates seem to differ between one provider and another, they're probably just aggregating over different banks, dealers and ECNs.
When the market is closed (or the bid or the offer price disappear) the current price is defined to be the last known current price, i.e. you act as though that event never happened.
Having read Google's disclaimer it turns out that currency prices are the indication of SIX Financial Exchange delayed by 3 minutes. As you can see on SIX's website, the indication is calculated by taking the best bid and offer out of 107 sources and then averaging over those two values.