WiFi

Quick question to all the intelligent and/or legal minded readers.

My phone is currently set to pick up WiFi signals automatically; there's regular places that I go with free WiFi access.

It's not unusual to see a car parked outside McDonalds on night duty for ten minutes whilst the driver checks their emails - the WiFi signal that McDonalds provides as a free service apparently stretches out to the car.

I remember recently a man getting nicked for parking outside someone's home address on a regular basis and hacking into their wireless internet access - but how does that compare to my phone? Quite a few times recently on crime scenes, hospital guards, constant watches etc, I've spent the time browsing the net.
On a couple of these occasions I've noticed when leaving the internet that I've actually been connected automatically to someone's private wireless access.

This is surprising in itself, simply because I'm shocked that there are still privately owned unsecured wireless networks around.

However, the point for me is this - am I committing an unwitting offence by doing this?

If so, exactly what..?