I work as a Radiation control tech. I'm the one that tells you to move over or stand back so you don't get microwaved lol. (I also keep people from eating uranium lol, but that isn't the point here.)

I've seen containers with rad material in them, behind lead-brick shielding (walls but no "roof") and my instrument reads more than what should be present after traveling through the shielding. I've heard this called shine (eg "beta shine" or "gamma shine").

I don't know why or how this happens, all I know is that I've seen it. Any explanations would be cool!