Currently a secret chat is indicate through a small green padlock which does not follow the style guidelines. Remove the current padlock and add a stock icon for that. Also the stock icon should be placed besides the other status icons in the list, as well as besides the profile picture.
Additionally, introduce a background color which will indicate clearly that this is a secret chat.
Flohack74
commented
7 years ago 
Currently a secret chat is indicate through a small green padlock which does not follow the style guidelines. Remove the current padlock and add a stock icon for that. Also the stock icon should be placed besides the other status icons in the list, as well as besides the profile picture.
Additionally, introduce a background color which will indicate clearly that this is a secret chat.