I'm not quite sure, were your tires "slashed" or, did they "mysteriously deflate"? Cuz it sounds like you've called it both ways.....if it's slashing, then that just sucks, I hope you find the mofo and beat him silly. BUT, if it just deflated, I agree with Gulzaker.....check the plug, make sure it's okay, and see if you can push it in/pull it out....reason I say this, is that a few weeks ago, after coming home from college for the weekend, one of the tires on our Blazer just went flat in the middle of the evening. So, I took it to Discount Tire (they'll check it for free!), and the guy put it in a dunk tank after he filled it back up, and there was no leak anywhere. Therefore, it wasn't a needle or a slash or a nail.....he said that someone had pushed in/pulled out (don't remember, sorry) the plug. It might've happened accidentally, or purposely, it's hard to say. (We live in a good city, in a good neighborhood, but it might've been done when the car was at college)
Either way, sorry about your situation, hope you figure it out.