It seems thieves have been infiltrating android powered devices to use them as a distributed bitcoin mining engine http://www.bbc.com/news/technology-26766045
Android apps that have been downloaded millions of times have been subverted to mine virtual coins for cyberthieves, say security firms. Two firms have found apps inside and outside the Google Play store seeded with the hidden mining code. The programs have been mining coins for the Dogecoin, Litecoin and Casinocoin virtual currencies. If installed, the booby-trapped apps will run down a phone's battery very quickly, said researchers.
Some of the apps harbouring the mining code were found on non-official Android stores but two of the programs, called Songs and Prized, are still available on the Google Play store. Songs has been downloaded at least one million times.
I think the old warning about there being no such thing as a free lunch seems pretty much the truth in this case