I've seen this happen on a laptop I was playing around with at BestBuy, about six months ago. In this case, there was nothing incognito or private about the browser settings... just a public computer.

I like to try searches on publicly available computers as I often see unexpected things... and this was unexpected.

I have noticed this also. Probably Google doing more testing or something.

Is anybody else noticing this?

Yep, I've been seeing this for ages when checking referrals with Statcounter. I can go through to the page where my site is supposedly ranked, usually on page 1, and it's not there, refresh the browser and I appear.

Now whether that has anything to do with the original person's cache and Google tracking I wonder since my browsers are kept extremely clean of all tracking devices wherever possible.

We know that different Google datacenters might show different search results.

Possible explanation to your question is that when you return back to page 1 you are hitting another datacenter which show different search results than the datacenter you hit in your first search attempt.

It appears there are some significant fluctuations in rankings this week in particular. Most of what we have seen is based in partial match domain keywords. I would expect things to level back out over the holiday.

Remember Cutts said a week ago that there was a multi-week rollout underway. It might be more noticeable during this period because the rollout might be at different stages across the different data centers.

>>different data centers... noticeable during rollouts<<

ZydoSEO and reseller - This in all likelihood explains the results I mention above (which turns out to have been more recent than six months ago).

Checking my calendar, I see that I was at BestBuy on the evening of March 22, 2013... and SearchEngineLand [searchengineland.com...] reports a Google announcement of the Panda 3.4 rollout on March 23. That's close enough for me.

I have been seeing this as well. Search for a term, site is NOWHERE to be found (except that we are paying for that terms via PPC as one of top results).
Then, go back to the page... not even an F5... just go back to the page and get difference results and we are ranked #1.
Is this a glitch? Or is there something more sinister? I doubt this is the case... but you'd think you have to at least consider the option that, if I am paying to advertise for a term then google will not show my site the first time people search to entice them to click paid links... then they start returning my site back in serps.
Sure seems like either a glitch or a revenue generation tool. Either one is troubling. This would explain why our rankings have not gone down, but our traffic has.

I've noticed this happen for at least a year now, but only very very rarely. Always assumed it was a glitch.