Why would my live in boyfriend's underwear keep disappearing?

When I did the wash today, there wasn't any of his underwear to be washed. When I asked him where all of his underwear are, he avoids the question and says They're around here somewhere " but they aren't here.
You are guys, what's happened to the missing underwear (at least 5 pairs)?
Why would my live in boyfriend's underwear keep disappearing?
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