Possible interpretations:
* Sarcastic farewell: They might be joking about leaving, or implying they don't want to stay longer.
* Friendly farewell: They might be feeling happy or lighthearted, and using laughter to emphasize their good mood.
* Relief at leaving: They might be relieved to be leaving a situation, and expressing their feelings with laughter.
Ultimately, the meaning depends on the context of the conversation and the relationship between the people involved.