Until the last couple of days: 

CD in drive, use Picard (2.12.3) CD lookup. 

Picard finds existant release/-s. 

Not to fiddle with search/release/ToC minutiae (https://musicbrainz.org/doc/Style/Release does not rule out ToC variants under same release), prompt-in own known release MBID to add DiscID. 

Result IF CD & existant DiscID match: MB warns that DiscID is already present, with that DiscID given in text highlighted red. 

 

Result since: No warning, no DiscID given in text, and “Attach CD TOC” enters loop. (To manually get/check DiscID from there, you have to back browser to earlier lookup result (e.g. “We used disc ID xW.ExaMLmp_UhJ4YZrgq471NlSs- to look up this information.”)).