Want an easy and inexpensive way to know if anyone’s been near your front door while you’re away?
Take two pennies and a quarter. Scatter them within inches of each other, in front of your door. If they ever disappear, you know someone has been there.
Most people won’t pick up a single penny; however they will pick up a quarter. Unfortunately, the silver color of the quarter usually blends in with the grey color of the sidewalk / concrete /porch. The bronze color of the pennies clashes with the grey concrete, and easily stands out, bringing attention to the quarter.
Likewise, use this trick on windows etc. Agreed, it makes no sense to find loose change on window sills, but the attraction of a quarter will override that thought. If you’re finding change missing regularly, it may be time to invest your next 27 cents into a more solid solution.