There is an easy solution to end shrinkage caused by putting merchandise out with the trash. You can institute, and make all employees aware of, trash removal policies. Inform them of the consequences trying to do a trash stash. Let them know they will lose their job and face criminal prosecution.
After all, a favorite ploy of a thief is to conceal merchandise, food or equipment in the trash. They then throw it in the dumpster and return later to pick it up! Generally, this type of theft will occur before a shift change or closing.
Institute random inspections of trash taken from the building. Check the dumpster area itself. Put the dumpster under lock and key. Set a policy that requires checking the key out and in. Restrict parking in or around the dumpster. This will force the thief to carry their loot further and thus increase their chance of observation.