I often see shady offers on the Currency exchange board, where someone is obviously looking for a fall guy to take the risk.
I've also seen offers to buy "old" or "fresh" Bitcoin, and pay a premium. No doubt this is either a direct scam, or someone trying to launder his money. If a naive but otherwise innocent victim falls for that, they become the main suspects.
I speculate yes, for plausible deniability the money mules are not being told everything about the operation. They only deposit, trade, withdraw and keep a certain percentage of the money. This might also be similar in casinos and sportsbooks also, where money mules are told to deposit, gamble, withdraw and keep a certain percentage.