Could more limited AI tech make a more damaging computer virus or cause an unexpected confidential data leak? Sure, but that's not the issue at hand.
You're free to disregard those, but I'm not sure Elon Musk is doing that.
The more damaging computer virus or data leak are only two of the possible worries. If a narrow AI simply helps black market chemists find more novel psychoactives than regulation can ever hope to handle, or if bots eliminate just 10% of jobs (say in transportation and retail to name just the most obvious) leading to massive societal unrest, or if they get better at solving captchas than humans are (which would lead to a massive crisis in anonymous communication and everything that depends on it)... all of these would make Musks prediction true in my book.
But these are just technological issues comparable to other mundane ones; just like how 3D printing could make it easy to create weapons, or how the rise of the automobile has created an enormous new cause of death and injury. There's not reason to think it would be outside the scope of ordinary policy-making methods to handle them.
Also, Solving Captchas is already pretty damn easy. A combination of algorithmic methods and crowdsourcing makes it quite cheap, especially for sites using older/easier captcha versions. Captcha is not a security plan; it's a speedbump that's getting easier to pass all the time (but still, no crisis will result from this).
Elon Musk submitted a comment to edge.org a day or so ago, on this article. It was later removed.
Now Elon has been making noises about AI safety lately in general, including for example mentioning Bostrom's Superintelligence on twitter. But this is the first time that I know of that he's come up with his own predictions of the timeframes involved, and I think his are rather quite soon compared to most.
We can compare this to MIRI's post in May this year, When Will AI Be Created, which illustrates that it seems reasonable to think of AI as being further away, but also that there is a lot of uncertainty on the issue.
Of course, "something seriously dangerous" might not refer to full blown superintelligent uFAI - there's plenty of space for disasters of magnitude in between the range of the 2010 flash crash and clippy turning the universe into paperclips to occur.
In any case, it's true that Musk has more "direct exposure" to those on the frontier of AGI research than your average person, and it's also true that he has an audience, so I think there is some interest to be found in his comments here.