Someone would need to not only have possession of your phone, but your password as well. So for a hacker to work this:

First, get your password. Second, find your location. Third, steal your phone, which for most people, is almost always on their person. Finally, crack whatever security mechanism you have on your phone.

For someone to go through all that trouble ... you must be storing some very valuable info. If that's the case, may I suggest that Dropbox is probably not the right platform? In fact, any internet connected platform is probably not the right answer.