Found this sign on a railway station platform. Presumably the fear is that someone might conceal a bomb in a saddlebag. Not completely far-fetched as the bicycle has a far from peaceful history - the Vietcong used loads.
Presumably, there is not an equivalent notice for the car park. That's probably because a search of a parked car is a more serious business. On the other hand, a car bomb could be very much larger than a bicycle bomb.
I wondered if the danger of a car bomb in station car parks had been thought of and perhaps stations were designed so that the nearest potentially explosive car was a "safe" distance from the platforms? A quick look at the satellite pictures on Google maps shows it isn't so. Although car parks are not right on station platforms like this bike park, some of them are pretty close.
So, this sign isn't evidence of a carefully thought out security policy, it's just a nonsense.