Only if your computer has the same workgroup name as another user (assuming you are running windows). If someone else has the same workgroup name, they can browse and potentially find your computer and view/modify the documents in your shared folder.
I personally like to name my workgroup on my computer to be something very random that no one else would have.

To change or view your workgroup name on your windows computer…
Click Start, click Run, type sysdm.cpl, and then click OK.
Then you can click the Change button to view and change your computer name. A reboot will be required.

Most importantly, there is no such thing as a secure wireless network with regard to residential standards including WEP and WPA, both of which can be defeated in minutes. With regards to an open network, you are even more vulnerable. You should disable the shares whenever you are in such an environment and ensure your firewall is enabled and properly configured.

most coffee place or place that have free wireless internet are safe
obviously any connecting computers (internet) has the possibility of one but no more risk then being at home

Ah Yep, so encrypt the folder