I've got one question. Does your wife usually keep a change of clothes and underwear in her work bag?

If this is the only time you've ever seen clothing in the bag and it just happens to be mystery lingerie then yeah, that's pretty damning.

If she normally travels with a full change of clothes or underwear then this may not be quite the smoking gun you think it is. You've been pretty vague about what they look like and "black bra and panties" could mean just about anything. If it's just underwear that happens to be black that you haven't noticed before that doesn't sound like a huge deal by itself.