I wouldn't know means I can't be expected to know that. It also infers some amount of resentment or sarcasm. Sarah "That cake was delicious!" John "I wouldn't know, you ate it all!"

"I wouldn't know" is very close to your other examples. However, it usually implies more than just a lack of knowledge. It also conveys an inability to obtain the knowledge such as not being on good terms with the person referenced or, in the case you present, not having seen the person in a long time.