In Voyager’s transporter room, there is an alcove on the side, and embedded in the wall is what appears to be a small transporter pad. I don’t believe anyone is ever shown interacting with this, so there’s no definitive explanation for what it is. Assuming it is what it appears to be, what would be the purpose of a very small transporter like this?
Perhaps it was not an expected hazardous sample / the transporter chief was not notified the sample was present.