Are you allowed to put google adsense ads on 404 /unreachable pages? Also, when you close a site down temporary to upgrade, can you put a google add near the message you are displaying saying temporary offline? Thanks!
The TOS, under the "Ad Placement" section states 404 Not Found, and 503 Service Temporarily Unavailable pages are not considered "content based pages", they are both considered error pages (404 being a client error, and 503 a server error). They are intermediary pages intended to give the user a choice on what to do in response to the error. There is no "content" to these pages, only "instruction". If you look at the note applied to that quote though, this does not apply to Adsense for Search, meaning you can put Adsense Search boxes on your 404 error pages to give users a way to find what they're looking for, and 503 users a chance to find something to keep them busy until the server is available again.