Using the autolist 2 tool (http://tools.wmflabs.org/autolist/index.php), I enter the WDQ query:
CLAIM[486:"D008180"]

which I think means "Give me the items that have MeSH id = "D008180" ".  It does indeed return an item with that claim Q1495661

but also returns items that have an no value as the object of that relationship, such as 
http://www.wikidata.org/wiki/Q3377653
http://www.wikidata.org/wiki/Q7796109
http://www.wikidata.org/wiki/Q1598365

Is this a bug in the query API or a bug in my query ?

thanks!
-Ben