There are many possibilities. My money is based on the fact that iOS 11 just came out. "Hmm... I see that there's a new iOS....Better update my personal phone even though I'm not using it at the moment...Wow! This phone is acting weird! Better take it to the security people at work."
I don't know that's what happened, but it's certainly plausible. If it is, it speaks well for Kelly's powers of observation (noticing that the phone was behaving in an unusual way) and ability to choose a proper course of action (taking it to the security professionals at the White House, rather than, as someone else noted, Geek Squad or whatever minimum wage clerk was manning the counter at the Verizon store).
> Then, how had he noticed it wasn't working properly if he wasn't using it?
How about any number of several dozen extremely obvious possibilities. A family photo album is on the personal phone and he wanted to retrieve it, so he used the phone and noticed something was wrong with it. He needed to retrieve contact information for a friend that was stored on it. You know, the same exact reasons any of us might fire up an old phone we still have.
I don't know that's what happened, but it's certainly plausible. If it is, it speaks well for Kelly's powers of observation (noticing that the phone was behaving in an unusual way) and ability to choose a proper course of action (taking it to the security professionals at the White House, rather than, as someone else noted, Geek Squad or whatever minimum wage clerk was manning the counter at the Verizon store).