While looking for web references I found the World Wide Words website. This is from its Ambient Advertising page.
… almost any kind of advertising that occurs in some non-standard medium outside the home. Examples are messages on the backs of car park receipts and at the bottom of golf holes, on hanging straps in railway carriages, on the handles of supermarket trolleys, and on the sides of egg cartons …Ambient advertising was in the news most recently when blinking lights were scattered around Boston as part of an advertising campaign. They were at first thought to be terrorist devices. Turner Broadcasting has agreed to pay a fine of $2 million.
Ambient advertising contains the seeds of its own destruction … because once the approach is copied and becomes commonplace, it ceases to surprise.
[Guardian, August 1997]
It seems to me that what was striking about these ads was not that they were ambient — ambient advertising is ubiquitous these days — but that they didn't look like advertising.