*Whenever one of the Titans - as many of them as still hang around the Tower, at least - care to check the lounge, they may notice a letter on the table. Then again, it's stuck underneath the bowl used to hold chips, so it might not be noticed for a while. When someone manages to not only find it, but also open and read it, they will discover that it's from Beast Boy.*
Sorry to spring this on you, kinda thing, but since it's pretty hard to talk to everyone at once without triggering the alarms... Anyway, dudes and dudettes, this is my official goodbye letter. Or at least, notice of a temporary absence. I'm going to do some other stuff. Maybe you'll see my name in lights some day!
I will totally come back to visit and kick all your names off the high scores on the GameStation, of course!
*There is of course a letter in Terra's room which explains matters in more detail, giving an address he can be contacted at, and saying a more personal goodbye. He wanted to do that in person, but what with everything involving Changeling, Robin, and Terra getting drunk and so on... well, the right opportunity just hadn't presented itself.
What there is not, anywhere in the Tower, is a Beast Boy himself. He appears to be long gone.*