Don’t you think someone whose bike has been stolen realises they should have locked it afterwards without you telling them?
No, not necessarily.
And, as the OP describes, there is the question of how they secured the bicycle, with what sort of lock, etc. There is no reason to believe that someone who’s had their bike stolen automatically thereby knows what is the optimal method for securing a bike against theft. (If they knew, they presumably would’ve secured their bike thus, and would not have had it stolen!)
No, not necessarily.
And, as the OP describes, there is the question of how they secured the bicycle, with what sort of lock, etc. There is no reason to believe that someone who’s had their bike stolen automatically thereby knows what is the optimal method for securing a bike against theft. (If they knew, they presumably would’ve secured their bike thus, and would not have had it stolen!)