a. Last Monday, I talked to her for the second time, in her office.
Is the sentence correctly punctuated?
Does it suggest that that was the second time I talked to her (whether in her office or not?
Does it suggest that the first time I talked to her might not have been in her office?
Isn't that what the comma suggests?
The sentence is punctuated fine, depending on the meaning.
I think there is a stronger implication that the first time might not have been in her office, but that's not certain here. All that's certain is that the second time occurred in her office.
If you wanted to clarify that both times happened in the office, you could reword it to:
Last Monday, I talked to her in her office for the second time.
If you wanted to clarify that the second time was in the office, but not the first, you could reword it to:
Last Monday, I talked to her for the second time, this time in her office.