i got 3.
but it isnt that easy to spot.
it doesnt really disturb you when you're playing,to be honest..just an imperfection(unless you have 1/2 of the screen filled with them )

lol, it's probably that you never notice or check it thoroughly when you bought it..

Lesson is..Get yourself the pixel tester mp4 from elsewhere and dump it in your MSPD, then bring it along when you are buying a new psp and load the mp4 and check..
I'm doing that next week when I'm buying my new Silver PSP..

But still, the dead pixel tester would help to reveal the dead pixel clearly..

If you really sure that it's zero dead pixel when checking on the spot..then it's your luck..

Well, this is my experience when i purchase my PSP from bernard.

At first, he did ask me to make sure the screen does not have any dead pixel. That is the first thing he ask me to do when i said i wanted to get a PSP.

However, he did state that dead pixel might come at a later stages, depending on luck, as his first PSP which is dead pixel free when he use it already have one now. 

the device only detects active wifi signals given by APs,AFAIK(if it's the same device we're talking about,anyway )
regardless if the device detects inactive signals as well or not,just play it safe and bring your psp with you.(why do you need to keep it in the car,btw?any reason? )