I bought this
freezer sensor with alert to keep an eye on freezer temps when I lost half of our upright freezer's contents due to the GFCI outlet it was plugged into dying. Some things that were buried stayed frozen (just barely) and tossed into the chest freezer (which is now an absolute disaster of a mess) and fridge freezers, other things got melted and given to the chickens.
I have a probe in the upright freezer and in the chest freezer and get an alert whenever someone opens one of them - I have the alert set to 5°F so that if a fault occurs, I have time to deal with it before anything actually melts.