They can only turn it on with a Court order. So there must be a case against you and a judge decides to listen to you to get evidence. They are not allowed to listen randomly into your mic. 
For something like that they have other technologies... 
The other is under the antiterrorism laws. 
Then they can also open your mic, but it always needs a judge to sign it. 
The thing they always do is recording calls and reading emails and stuff. But here it is not allowed to be a human, it is a computer. On special words it makes an alarm and they have to listen to it... I don't know the actual trigger words tho. 
I think to use that against you, they also need a judge to allow it.