I think its because of your jailbreak. Because my phone just started randomly doing it today and I know I didn't get any water in it.
I had it plugged up into my car and when I turned my car off it popped up. Then I just tried playing music from my phone without it being plugged into anything and it still kept popping up.
When I tried playing music there would be no volume button so I wouldn't hear anything.
iPhone 4, Jailbreak.