Why does my laptop emit electromagnetic radiation when charging (only)?
You only detect it when charging because you are using a toy of a radiation detector.  

Your laptop is generating electromagnetic energy at all times (when operating.)

It will probably generate more (at certain frequencies) when charging.  Your detector can see them better - probably because the laptop begins emitting a frequency the detector can see, or because emits more of some frequency that was too weak before.

If your detector can't see it, that's more a problem with the detector.

The detectors you can buy on Amazon and Fleabay are more in the line of "separating worried but unknowledgeable people from their money" than "accurate measurement of electromagnetic intensity."



That thing definitely belongs in the class of  "separating worried but unknowledgeable people from their money."  

No mention of the units of measurement, the "specifications" don't match up in anyway, and there are more pictures of the box than the device.