Often times we bump into wireless networks around our homes, offices and recreation centres and quickly connect and start uploadingz, downloading and updating apps. As Christians in modern society can this be regarded as sin.? What's role should a Christian play in such a situation.

Someone opened his or her WiFi for all to use knowing fully well that it should be password, you called that a sin? No. But if you feel uncomfortable based on your conscience, you can as well tell the owner that you used the WiFi. If the owner wasn't aware and he ask you to pay and you refuse to pay then it is a sin.
If you hack a site to browse freely, you had committed a sin hence you must restitute if not you will pay dearly for it.