How to determine what block isn't covered here?

I have a block of code like this (class/property names have been changed to meet company guidelines):

if (output.ExecutionStatus == RuleExecutionStatus.Success &&
    !obj.ListOfObjs.Cast<ConcreteType>().Any(p => p.Id == o.Id))
{
    obj.AddConcreteType(output.ConcreteObj);
}

Before adding && !obj.ListOfObjs.Cast<ConcreteType>().Any(p => p.Id == o.Id) this line was 100% covered. I added one test, called Test 2 down below.

So, after adding the new test, this block is almost 100% covered; but I can't figure out what isn't getting covered. It states 1 block is not covered. The tests executing now have the following attributes:

• Test 1: returns success, has 0 in ListOfObjs, no match, adds successfully.
• Test 2: returns success, has 2 in ListOfObjs, 1 matches on Id, does not add.

What I've Tried

• Adding a test that has 1 in ListOfObjs, no match, adds successfully. I thought maybe the issue was surrounding the fact that it didn't cover the Any case where 0 matched.