Both kinds, really. If someone takes benefits for which they really aren't eligible (and who judges that, right?), that's not that different from a doctor over-billing charges, or delivery services not delivering as promised. My Father in law was on O2 for a few years before he passed. The number of empty or partial bottles he got delivered was eye-opening. Never enough to make a difference to HIM, but consistently done over a number of patients, it adds up. I don't think he was unique.
$2.4B of recovered fraud on total medicare spending of about $650B implies a fraud rate of about 0.3%. I don't have proof, but I bet it's actually a lot higher fraud rate than that. If $400M of fraud investigation costs yields a $6-$1 ratio of savings, we should be spending a LOT more on fraud prevention.