Why source is empty object for hook_feeds_after_parse in dpm()?
Ooops ok,
dpm($source) does not show protected class properties if you don't specify which protected variable you want to display.
You have to point the property you want to display.

After some research in FeedsConfigurable.inc and DB 
dpm($source-&gt;id) is the unique id (string!) I search for.

/n