I love stuff like this: there's actually a technical word for that (and it's probably the least-interesting one around) but the word you're looking for, literally, is "cloaked."  It's a bit like the Klingon/Romulan cloaking devices in the Star Trek universe, which, to borrow on other elements of this thread make use of a "nobody-here-but-us-chickens field"...basically you don't become invisible so much as you're easy to overlook...