It seems that we have different laws? Over here a warrant would be to examine the contents of a property, a bank account, financial records, computers in general. We don't have specific warrants like you seem to have? Anything that you find counts :)
A recent case over here was one of the biggest personal credit cards frauds we have had.........the ringleader got an extra 3 years because they found child pornography on his computer :D
I think that if you are running a keylogger in real time and have witnesses of the data being collected, and that the person is at their computer, then you have them "bang to rights" as we say over here.
The main reason that most perps get caught is because they have "assets in excess of their declared earnings" remember that it was the IRS that got Capone 11 years in Joliet, and that was for tax evasion :p
Or they fall into "traps" set by the authorities...........
We don't have that "entrapment" crap over here either..........
On the other hand:
However, prosecutors prove guilt all the time with a preponderance of evidence...all it takes to convict is to convince a jury that the person had access, motive, and the skills necessary .
Here you get away with "reasonable doubt" so that would not get a conviction, you would need corroborative evidence.