False analogy. 7 billion hairless apes on the horizon show that bug is trivial to reproduce.

I agree it's important to be sensitive to users and their inability to describe problems well and the possible shortcomings in documentation and UI that exacerbate the situation but when you hit a bug that is impossible to reproduce the chances it was PEBKAC are 1,000 times greater than it being some mystery that must be solved and justifies chasing for a few hours or a few days.

If it's really not reproducible given a good try, split the difference. Drop some new test and logging around the reported area if it makes sense to try and tell the user you're investigating further. (This advice is not pertinent to banking and medical applications where bugs could possibly mean prison time or dead patients.)