What's that Smell?

I took it out, loaded up the batteries, put in some peanut butter and set it in place. It sat there for a few weeks and nothing happened but I began to noticed a bad smell in the family room, just couldn't trace it. One evening the alarm went off on the trap and I went to investigate. Inside was a recently killed mouse and also a rotting one. It seems the first mouse was killed but never set off the alarm. I guess I'll be checking them periodically to ensure this doesn't happen again.

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