Google recently admitted to running a "small scale experiment" in including vacation-rental listings in its hotel search returns for major cities in France, Spain, Italy, Germany and Portugal.

Booking is handed off to Booking.com.

If this initiative goes forward, that will position Google as a serious competitor of Airbnb, HomeAway and other "sharing economy" lodging aggregators.

And as in every other market in which it competes, Google has the advantage of reaching shoppers who have expressed an intention to buy.

This could get interesting.