If you look at the area around the front IR sensor, you can see that there is a hole in the chassis for the sensor and rubber gasket to fit underneath the body grip material.
The K-5 and K-3 have similar looking chassis around the area of the IR sensor.
If you press on the sensor, and you feel slight movement, you are probably just feeling the rubber gasket move a bit. When I press mine, I can feel about 0.01 mm of depression (at 15 degrees Celsius ambient). If the body grip was warmer, the gasket rubbers and grip material might be softer, and you might feel more movement, I guess.