We've recently migrated an old custom cms site to Umbraco at the same domain.
Google Webmaster Tool is reporting 1000s of "Not Found" errors from the old 404 page in the format error/404?aspxerrorpath=/path/to/old/url.
Does this mean that Google has historically indexed all these pages (ie before switching to Umbraco)? Searching for these urls produces no results.
Do we need to take any action (such as 410'ing pages which match this pattern) or will the new site returning a 404 be enough?
Does this mean that Google has historically indexed all these pages (ie before switching to Umbraco)?
Not necessarily, although it would appear to suggest that the old site (incorrectly) redirected to the error page, rather than serving it directly (as with ErrorDocument etc). And Google has "helpfully" remembered this.
Do we need to take any action (such as 410'ing pages which match this pattern)
You don't need to do anything. There's no SEO penalty for having a stack of 404s if these are indeed genuine 404s.
However, it can be annoying to have Google's Search Console (formerly GWT) report polluted by all these 404s, since you might miss the ones that really matter. So, you could return a 410 Gone instead - this sends a stronger signal to Google that these URLs really are gone and not coming back, so in time they will hopefully be dropped completely.
