I've recently come into ownership of two very nice bikes. I live in an okay area, so I haven't had to worry about bike security very often.
We do, however, often take our bikes to less than savory areas - we have a Thule T2 Pro and often stop for lunch ~wherever~.
What is the most reasonable way to secure our bikes from theft? I understand that no security is perfect. (If you see my profile, most of my rep is form Security.SE /humblebrag)
Presently we do this - this is assuming a worst case, run down terrible area, with a bike chop shop next door to where we park, etc etc.
1) Avoid parking in such areas.
2) Stay with the bikes if at all possible.
3) Bikes are attached to the Thule T2 Pro using the built in cable locks - it connects the front wheel and frame to the rack. The T2 Pro can be firmly attached to the trailer hitch using an expansion screw included in the design of the rack.
We are also considering adding the following:
1) A cable lock threaded through the seat, frame, and rear wheel, to complement the front cable.
2) Locking the bikes together using a flexible plate-link lock.
Neither of these are absurdly harder to implement, and avoid some, but not all, piece-meal theft.
EDIT: The T2 can be attached to the car in such a way that it can't be removed without the key, or spending some time 'hacking' the locking mechanism in one way or another. This post originally didn't say that and some comments will be confusing due to this.