From http://tickets.musicbrainz.org/browse/MBS-2529

An easy way to tell standalone recordings apart from ones created by a release:

A standalone recording will have an "add standalone recording" edit with a mandatory note (eg, http://musicbrainz.org/recording/1ba9fe0e-aab7-4151-bcf9-ce5b7fca7fd1/edits).

A recording created through the release editor will not have any history at all (eg, http://musicbrainz.org/recording/70fab960-184a-46de-9d0e-4559b573c3a0/edits).

So, ModBot should be set to purge recordings if...

1. There is no "add standalone recording" edit in the recording's history...
2. It is not attached to any releases/tracklists.
3. There are no open edits (edit/merge/remove recording)

It'd be great if this could be added soon because I just realized how much of a mess this is causing. (I want to vote down someone's addrelease right now, actually, since spelling errors and no track lengths, and some pre-existing recordings should have been used. There are 32 recordings involved and 32 orphans will be painful.)