She might not have had time to dispose of it at the time she was carrying out her plan, but she could have retrieved it and disposed of it the next day.

...hmmm....shades of Dr. Kepple in Double Exposure not getting rid of the calibration converter..

...one of my favorite episodes too.

I just thought of a 'double' meaning for the title; In the end Columbo not only exposes the murderer, he exposes the giggling, mischievious child behind the intelligent, analytical Dr. Kepple. Double exposure indeed!