There is an interesting paper from Georgia Tech College describing a clever proof of concept where a phone is used to eavesdrop on keystrokes.
This is done by leveraging the phone motion sensor capability and placing it next to a keyboard. They managed to create a dictionary of words/vibrations that is able to recognise words typed on a keyboard just by analysing the vibrations made from typing.
Of course, you are likely to notice someone’s else phone sitting next to your keyboard but what if your phone got hacked and that software loaded onto it?

They conducted their proof of concept on an iPhone 4 but this is likely to be also possible on other platforms/devices.

In fact, with upcoming smart watches this concept will be even more relevant! Now I can see a use for that Apple M7 chip! ;)

As I am typing this note, my phone is next to my keyboard. Maybe I should move it away…