I agree with you, if he adds a negative number to the array, the code will give an erroneous value

the instructions say that "The numbers will also all be unique"

so you'll never get two of the same values, e.g. two '3's in your example

I do not think that this solution is correct.
It will overlook same-as items.

Try this variable with it.
numbers_list = [1, 2, 3, 3, 6, 7, 8, 9, 10, 12]

"3" would be the first non-consecutive item.
But the above function will choose "12"

The check should be whether it is consecutive (+1) or not.