lol! I assume you are a somewhat reasonable person. Lets say this happened to you and you really had no idea where the panties came from. Same shit in the past too (wife caught you talking to exes, suspicious activity on computer), but say you never even cheated. You could see though how it really didn't look good to your wife for you to be talking to exes. Hell, I'm sure you wouldn't like it if your wife was talking to her ex boyfriends.
Now back to the finding of the panties and her questioning you. I would think that given the past you would want to reassure your wife completely that you had no idea how they got there but that you would do your best to find out how they did.
Or am I wrong about you being a reasonable person?