Since cs137 emits gammas, your assertion is wrong. A standard gamma detector would detect gammas thru a can. batas no. gammas yes.
Since you’re being picky, CS137 does not emmit gamma rays when it decays, but the decay product BA137 does, and yes you would be able to detect gammas through the can, but there would be some attenuation, although not more than a couple of percent max given the thickness of your average can. So the maximum accuracy would be to measure the tuna out of the can. Further a reading of .05rad is much higher than you’d expect even if the tuna were contaminated. I suspect that the previous poster’s meter is in need of calibration.