Yes, your data is most likely to get wiped without the person trying to access it.
Once they pull power, turn off the phone, place it in a foil bag, etc. not much you can do. And anyone with half a brain will do that when taking a phone.
On your end, as you mentioned, change all your passwords that you can think of that the phone has in its memory - starting with your google account. Probably more of a precaution, but it will give you peace of mind.
Also, consider replacing any credit cards that may be on that phone. That is a major hassle, but that would just be more peace of mind for you.