Dangers of Apps running in "Hidden mode"?
1) Are there any similar loopholes that can be dangerous?


They are not actually loopholes. They are ignorance of the user. For any Android app to be able record your calls, needs these permissions: (This is a code snippet from manifest XML file.)

&lt;uses-permission android:name="android.permission.READ_CONTACTS" /&gt;

&lt;uses-permission android:name="android.permission.READ_PHONE_STATE" /&gt;

&lt;uses-permission android:name="android.permission.RECORD_AUDIO"/&gt;

&lt;uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /&gt;

While installing apps you will see something like this on Google Play: Permissions: Read Contacts, Read Phone State, Record Audio, Read and Write to External Storage. (Something like that.)


  2) What should I do as an android user?


So, when you install an App, you need to carefully see what permissions the App is asking for? Is it really needed for what the app is supposed to do? All you need to do is be careful when installing an App, see that you don't grant too many unwanted permissions.


  3) What has Google done/planning to do to prevent such apps in Google
  Play?


They have already created permissions! No app can access things outside of what permissions it has been granted!

Hope this answers your doubts... :)

PS: Here's story related to your questions. There are already things which are recording calls.