If you do, don't every let her know it is there, and don't use any evidence from it in court. If you find something out, go to wherever and initiate some other type of surveillance or find some other proof to back up what you find.
It might be that she just likes going to the mall. Or maybe she's a reader and spends a lot of time in the library. Maybe she has some girl friends and the play cards. Don't assume too much without any facts.
Some people are just very private with their personal time and don't see why other's need to know every single detail. This can place a huge burden on a marriage, even one that might otherwise be able to be saved or reconciled.
What has been Seen, Cannot be Un-Seen!