Once the recipient name logs in for the first time after you sent a memo, the memo disappears. If someone other than the actual recipient is logging in with their nic before they do, the memo will not be there for them when they actually do log in.

I can't imagine any hackers being dumb enough to leave a clue like this, but then some of us haven't changed our passwords since before 9/11, so how dumb would they be relative to myself and others.