It seems perfectly natural and no-risk: sweeping your hand over the top of your unit’s back door to brush snow away or bracing your palm over the weather strip there as you guide a prisoner in or out of the back seat.
But—consider this:
A police department mechanic in South Portland, ME, discovered a single-edge razor blade—cutting edge up—embedded into the top of the back door weather strip on one of the agency’s cruisers. It appears a subject positioned the blade there so an officer who didn’t see it would slice his hand when he placed or moved his fingers or palm over it.
Even the “safest” areas warrant a cautionary glance.