I am writing a short story in the present tense. Which line of dialogue is correct? 'When she walks in, carry on as though we haven’t noticed she’s been gone.’ 'When she walks in, carry on as though we hadn't noticed she'd been gone.' Also is it 'if' or 'though'?
When she walks in, carry on as if we hadn't noticed she had gone/left. That would be my choice. Even in present tense I would place the noticing in the past.
As forkfoot said, dialogue bends the rules of tense, because dialogue can be about things that happened in the past, future, or present, when the actual story could be solidly in present tense. My version of your suggestions would read: "When she walks in, carry on as though we didn't notice she was gone." If you need to use one of your two examples, I'd use the former: "When she walks in, carry on as though we haven't noticed she's been gone." My reasoning here is that with the usage of "when she walks in" you are pushing the dialogue to present tense, "walks" being the present tense verb. When this happens, we will do this. It does not make grammatical sense to put the second half of the sentence into past tense: When this happens, we had done this.