Wanting to go onto a computer and not leave a trace requires some mad computer skills. And next to impossible to do in real time. Let's take a look at who's tracking me as I type this. It logs on my computer, which is under my control and somewhat easy to deal with that log. But the company that is providing me with internet service is also logging this. Every time I upload and download is recorded. I have no ability to destroy that record without getting into their system. And the internet service provider (which can be different than who's providing the internet connection) also records this.
This is all part of how they catch pedophiles.
So if the message is sent in real time, you can back trace through those levels and you'll find traces. It's not very hard to do if you have the appropriate skills.
To get around this, because you can't destroy the tracks that are left, you have to make this complicated. And this is hard to do without seriously mad computer skills. (Hackers are very smart people.) Start thinking spyware and malware.
First you have to get a route into the computer you want to hack, let's call it Popeye's. Now Popeye isn't stupid, so every night he shuts off his computer. However, that's an easy problem for a hacker to solve. Either you turn the computer on remotely, or you wait for Popeye to wake up in the morning. In your case, waiting for Popeye to wake up is the better route.
Now hacker, at let's say three in the morning, sends a bundle of information to Popeye, probably using Popeye's email. Hacker routes it through multiple ISPs throughout the world, faking the computer's ID. In this bundle is the message from the hacker, as well as a couple of other things. This bundle just sits around waiting for Popeye to wake up.
Popeye wakes up, the bundle is sent, but hacker being really smart, has programed it to not activate for a while. Popeye may see an email advertising a bigger dick for him. The bundle sits around on Popeye's computer, until either a certain time or a certain action by Popeye.
Then the hacker's message pops up, claiming priority over all other programs presently running on Popeye's computer. Popeye sees the message (which can be a video recording). Message lasts for let's say twenty seconds. Now for the real fun part. The bundle contains a program that now destroys the message, and any trace of the message disappears. A really good hacker actually will have the program fill in the twenty seconds with another activity (downloading kiddie porn surfing would be a good touch).
Finding the bundle from the hacker is next to impossible, because of the time separation. The IT guy trying to trace this doesn't know where to begin.
And as I've said, mad computer skills are needed. A friend of mine had this happen to one of his clients. Took over a year and a half to find this and nearly ended in the client's divorce (the hacker was sending him emails from the client's alleged "girlfriend," including nude pictures of a woman, all designed to show up on the family's computer and be seen by the wife). Hacker felt that he'd been screwed in a contract by the client.
Best of luck,
Jim Clark-Dawe