Most cafes it would be difficult to steal a bike from without people noticing.
They might notice but not realise that something untoward was happening! I have heard of cases of thieves dressed as cyclists walking up to bikes, climbing onboard and riding them round the corner to a mate in a van.
When we stopped at Eureka on nickyboy's Llandudno ride there were rows and rows of unlocked bikes with their owners in the cafe or out the back. It would be simple to have made off on one.
A bunch of us rode to an idyllic country cafe. It was a former farm house down a tiny lane, chickens wandering about between us. Bikes and cyclists everywhere. About a week later some low life got on somebody else's bike in front of witnesses and just rode off on it.
It's all about cheeky confidence. If you act as if it is your bike then people will think that it is. My mum used to work in a department store next to the TV department. Somebody in a white coat walked up to the display, unplugged a TV and walked off with it. He returned a few minutes later to get another one. He kept doing it until
finally challenged by a security guard on the stairs. He threw that set at the guard and ran to a waiting van, escaping with thousands of pounds worth of sets.