Not particularly something we've had a request for in the past, nor do I really fully understand the exact use case. You may want to create a thread in the Suggestions forum and explain in more detail then we can perhaps consider it for the future.
I'm gonna have to ask why you would want this? Wouldn't you want your mods that were online to know if you were there in case they needed to ask you anything? Hiding from your staff isn't good. It's almost like the non-existent owner of a forum that I admin on. He's pretty much useless if he is never logged in and taking care of the forum like he should, 6 months and counting. Thats just my 2 cents.