It is no longer safe to leave a bike outside / locked with a quality D lock / in a 'secure' cycle park / locked to a Sheffield stand etc etc. Police advice on this is completely irresponsible. This is probably because the police are so uninvolved in bike theft that they know nothing about it. The only rare occasion you may get your bike back is if police have raided a site which has stolen property and in order to up the charge they type in the frame number of the bike and find it matches a reported stolen bike. That may happen to about 1 in 100 stolen bikes as a guess.
As a minimum a bike should be locked inside a home or inside a high security bike garage/locker. Bike locks are quickly defeated with portable angle grinders and this takes seconds.
Commuting, Day rides, Audax, Incidents, etc.