In fact rebels has bragged they’ve overrun an air defense base capturing two Buks in late June, though it was reported both were in bad condition.
This is generally regarded as disinformation, and promoted by the "separatists" themselves who like any bit of propaganda to raise morale from their ignorant followers. The "reports" all go back to a single article by Russian state controlled media, and have not been confirmed by any other source. The Russkies have a habit of planting such stories into their media right before sending in the "captured" item from Russia into Ukraine.
Furthermore, NATO knew about the Buk launchers 2 weeks ago, and saw them on the Russian border, with training going on for your so-called "plug and play" weapon, with Russian officers assisting "separatists". This was reported about 2 weeks ago in a press briefing, but no one in the media seemed to notice until the last day or two.