The thief steals the bicycle if he opens the lock, that is, if the code he tries is the same as the code for the lock.

Test 1:
Den's keys: 0001
            0000

           1st night   2nd night   3rd night  4th night  ... ... ...    All nights

bike's key 0001        0000        0001       0000                      -

bad man    0000        0001        0003       0004                      -

result     no          no          no         no                        no

Test 2:
Den's keys: 0002
            0001

           1st night   2nd night   3rd night  4th night  ... ... ...    All nights

bike's key 0002        0001        0002       0001                      -

bad man    0000        0001        0003       0004                      -

result     no          yes         no         no                        yes

In the first test all nights result is "no". That why in the first test result is no.