Using Op-Amp to form a uni-gain buffer, what happen if IN+ is floating?
Figure 1. The setup.

Since this is a voltage follower circuit, the output voltage will be whatever the input voltage is. 

The datasheet says that it has


  Low Bias Current: 15 pA (Maximum)


and an input impedance of 100 M&Omega;. That means that it will be very susceptible to pickup of stray voltages and depending on how much of an "antenna" the input is connected to it will pick up everything from mains hum to radio wave signals. The output will follow whatever the input sees.