Methods to detect errors in cache
Modern processors don't have ECC. However they do have about the same error rate:  


A modern 7500 Xeon with 24MB of L3 cache has about 0.2 or 0.2 error per year.   
250GB of DRAM would have about 0.02 to 0.2 errors per year
Source


Physical construction of the memory cells and number of transistors has a lot to do with it. Computers that are lower in elevation or shielded from radiation experience less errors. 

In a computer system if more redundancy is needed, there are fault tolerant algorithms that can can be run in software to check code multiple times to ensure that no errors have occurred in the hardware. For example, if there is a calculation, you could run it three times and then store it, the values are then checked to see if the result is correct.   See Fault tolerant computer system