Problem:

An interesting tidbit floating around the internet these days is that Dr. Duncan MacDougall apparently weighed people shortly before and after death, and found an average of a 21 gram discrepancy, to which he attributed to be the soul departing. As we all know, this certainly can't be the right explanation. Putting aside likely extreme bias and instrumental/experimental error,

Question: Is there any bodily process at death that could potentially explain a small decrease in observed mass? Please describe.