I was told that story by a friend. From what I have seen some of my colleagues do I'd say the story is very likely true.
A thing that really happened (at a tax office in Finland, in 2004) is even more unbelievable but it was big in the news back then: a man had died at his desk and they noticed it only on the third day.
The cleaners thought he was staring at his computer, concentratedly, and his colleagues didn't miss him at lunch because he often worked out of house.
Found a short summary: http://news.bbc.co.uk/2/hi/europe/3410547.stm
Such a thing could never happen at my office: here people greet the cleaners and chat a moment with them
It'd instantly be noticed if someone didn't reply.