Why does this detector detects wifi signal as well?
Here's the circuit: -



And not surprisingly it will detect a wide range of radio frequencies because it doesn't use tuned circuits to "focus" on transmissions that are nominally cellular in nature.

It works because the LM358 op-amp is susceptible to virtually all RF energy above a certain signal level when a pick-up antenna is attached to its inputs. Normally, the bandwidth of the LM358 is about 1 MHz but the cellular transmission is round about 1,000 times higher. Please do not think that the LM358 is actually officially doing something that is implied in the data sheet - it's forcing the LM358 to operate well beyond its normal use and the input transistors are turning into rectifiers and AM demodulating the celuular transmission.

Virtually any modest to low bandwidth op-amp would do the same.

A better circuit (i.e. one that is not blowing smoke in certain areas) could be made by a simple germanium diode detector to produce a DC level proportional to incident RF energy received and then use an op-amp conventionally to drive the LED. You could even take this a step or two further by adding passive tuned circuits to at least make an attempt to tune it in the correct area.

I think Seattle University are using "shock and awe" methods to get new students! Not impressed!