tedster - 5:21 am on Feb 8, 2011 (gmt 0) [edited by: tedster at 5:32 am (utc) on Feb 8, 2011]
If it's while people are typing in the search box with instant on...
And it is - if there's a three second pause in typing.
Also, as TheMadScientist emphasized above, Adsense impressions have absolutely nothing to do with SERPs impressions - two very different kinds of "impressions". Adsense needs your actual page to load in the browser. SERP impressions just need your URL to show up in a search result. It seems like that might be the distinction that's been throwing your stat interpretations for a loop.
[edited by: tedster at 5:32 am (utc) on Feb 8, 2011]