Even as a permanent resident, when entering the country it's standard procedure for Immigration to ask you why you left. Here are the last two conversations I had:
Customs and Immigration, Detroit Metro-Worth, June 2005
Immigration: What were you doing in Japan?
Moi: I'm an anthropologist, I was doing fieldwork with people with disabilities.
Immigration: Disabilities? Like me?
Moi: [Peering over counter at his legs. He isn't in a wheelchair, no braces on his legs. He is kind of overweight... I give him a quizzical look]
Immigration: [Turning very red, he mumbles] umm... I have a bad knee.. [he mumbles some more, then STAMPS my passport and hands it back] Welcome back.
The kicker, though, was the last time I came through:
Customs and Immigration, Detroit, Metro-Worth, April 2005
Customs: What were you doing in Japan?
Moi: I'm an anthropologist.
Customs: [She looks at my large suitcases with a worried look] You don't have any bones in there, do you?
Moi: Bones? ... Oh... no.... I'm a cultural anthropologist not a forensic anthropologist.
Customs: [Looks even more worried] You have bacterial cultures?
Moi: Hurriedly, No, no, no, I study human cultures. Like Japanese culture.
Customs: OK. Welcome back.
Well, I won't make any bones about it, but at least they didn't think I was a spy.