The sandbox is not just for new domains, its for new pages on established domains as well. A site of mine indexed since 1997 with 100k pages and number 1 rankings across the board has just expanded into a slightly differnent niche. Without going into details its as if a golf equipment supplier started to sell golf insurance on their site. The subject is similar but different. Anyway the new page was listed and got page 3 rankings for the keywords after a couple of days. Over the last 2 months its been climbing a spot or 2 per week and its now at number 11. There has been no SEO work or marketing done for the new page other than a sitewide navigtion link on my site. The only explanation is an ageing delay on the page or the links pointing to the page. Interesting to see this on an established site though. Has anyone else seen this?
That is not indicitive of a sandbox effect. Coming out of the sandbox is supposed to take longer and be more of a bang. Looks like normal aging/backlink gathering progress to me.
But the sandbox is an ageing delay applied to domains (and pages) and the backlinks they have. It would make no sense for google to apply the sandbox as a simple 'on' or 'off' filter and this is certainly not how I have experienced it in the past with new domains. I've never seen a site that went from sandboxed and no rankings to top rankings in one move.
It's certainly not the only explanation. Keep in mind that Google is undergoing a major update to their search engine right now. Results have not been changing normally since February or so. So it could well have been delayed as a byproduct of that.
That has always been my experience. You go from not ranking at all to top 10 in a single day. Google also grandfathered older domains from the sandbox as long as there wasn't any ownership change, so you aren't experiencing sandbox problems unless you have changed your registrar information.