PH
Before severe weather or a possible power interruption, freeze a small container of water and place a coin on top of the frozen surface.
If power is lost while you're away and the freezer warms enough for the ice to melt substantially, the coin can sink. If the water later refreezes, the coin's changed position provides evidence that significant thawing occurred.
It isn't a substitute for proper food-temperature monitoring, but it can indicate that the freezer didn't remain continuously frozen.
When food safety is uncertain, follow official food-safety guidance rather than relying only on appearance.
© USDA