It doesn't take much thinking to take it to the police.
If someone just left 30 thousand in your taxi wouldn't you think the person would want it back? And surely it wouldn't be so hard to find the taxi driver.
What's beyond me is how someone can forget 30 000 in a car, if I'd have this kind of money with me, surely I wouldn't just 'forget it'.
Conclusion, taxi driver did the only thing he could and got a huge pile of money.