Locks

stating obvious There is no lock that is undefeatable. I had an Abus Granit X-Plus d-lock (£50) - one of the very highest rated on the market - and came out of my house one morning to find it in pieces & the bike gone. It had been cut through like the proverbial knife through butter. The bike was locked up in the front garden directly beneath our bedroom, so whoever did it was very quick and quiet. So it's all relative.

almost without doubt this will have been done using hydraulic cutting equipment. it's pretty much silent, doesn't require power (the hydraulics are pumped using a lever like a car jack), and it's very powerful.
these things are scary, as you say there's nothing you can do to prevent a thief who's carrying one of these from stealing your bike.

be very afraid.