[judicial meanz]

You can scan a lot more than four floors. Radiation has an effect called "groundshine" that makes it radiate in 360 degrees from its point of rest.

An object located in a 4th floor apartment would "shine" radiation up, down, and to the sides. Once you get a "stray" gamma reading, you simply triangulate it to the source.

Its easy to tell "stray" gamma from radon and naturally occuring radiation. You look for a spike among natural radiation backgrounds, and then narrow the spike down to a point of origin.

Remember, however, that you are probably looking for something barely above background level and its a treacherous thing to pick out unless you are really good at it.

[milkncookies]

Repost:

Because of the large amounts of naturally occurring background radiation in urban areas — emitted by everything from granite and ceramic tiles to cancer therapy machines — locating a bomb's radioactive signature in such settings, "is like looking for a needle in a haystack of needles," according to one NEST searcher.

[JustPiper]

So these devices Sean is referring to, the law of average would dictate that some would be found