Strange input

I ordered a 37 in 1 sensor kit for arduino online and it arrived today, so I tested some of the sensors, connected them to my arduino uno and they worked pretty good. Now to the strange part: i wrote a little programm in processing to visualize the values which I'm getting, after I disconnected the sensor I noticed a sinus-wave like form on the screen. I still had the cabel on pin A0 connected but there wasn't a sensor which could give out such results. I have no idea where this wave is getting from but I also noticed that this wave has a frequenzy of about 0.1 to 0.3 Hz. Please explain it to me if you know something about it. I will add the code of the programm and some pictures later. Sorry for my bad english.

Nothing to worry about; an unconnected input is also known as a floating input. The cable is acting as an aerial and picking up noise.

Yep - entirely normal. The input impedance of the analogue and digital inputs is extremely high, so a floating input will pick up all sorts of things.

Usually in my workshop it's the high frequency noise from the nearby fluorescent lights, plus a lot of 50Hz hum from the mains when I move my hands anywhere near the circuit.

A 0.1Hz to 0.3Hz signal is an unusually low frequency - it would be fun trying to locate its source. I can't think what it could be at the moment.

Hi, I can only agree!! Like Steve I see quite a difference when I switch fluorescent lights on or off, there well known for their noise!! and readings change as you move your hands about!! Try it, it can be fun, ground yourself on something the a grounded line....

So don't have any floating inputs, tie them to ground or 5V (with a resistor) depending on what your doing with them and what you expect the input to be.