I had never made a reservation, never even searched for a hotel, as we were staying with relatives. So, there is zero chance that it had anything to do with other searches or reservation attempts, and the flights were made directly at the airline site (i.e., not through Google Flights or a travel aggregator).

The advert populated in the same time-frame as the flight; the airline's flight confirmation email to my Gmail acct had clearly been was parsed, and there was nothing in the email having to do with a hotel.

Moreover, when I clicked thru and found the menus, Google settings pages were there, offering to enable/disable the feature, along with describing that the "helpful" auxiliary reservation feature could not be turned off separately, and providing a feedback opportunity.

My recollection is that this advertisement itself also came with a google message about how this was a helpful new feature to complete my travel palns.

Obviously I turned it off, and slammed them in the feedback, in particular how I couldn't manage the sub-features (actual vs advertised reservations) separately.

Neither item ever happened again, which I would expect having turned off the feature. Which also indicates that this was something under Google's control as opposed to an infection or bug.

My guess is that this is a short-lived experiment, gathered a lot of the kind of feedback I gave, and was cut off by one of their smarter managers.

1) This occurred in exactly the same set of minutes since the email direct from the airline arrived and Google parsed it and posted the flight info to my calendar

2) The hotel locations and dates for the fake reservation were exactly matched to the airline travel dates.

It is also possible to win the lottery, but to post such an exactly targeted advertisement is for all practical purposes impossible without access to the email. Only three parties had access to it: The airline, me, and Google.