A "lost stop" is actually in reference to the junior firefighter, also known as a probie. The probie earns this nickname as they're often used to perform important, but menial roles on the fireground. Sometimes this involves holding the doors open in place of a door chock or "door stop". So when the captain comes on the portable radio to announce "lost stop" - it means that the probie assigned to door duty has become disoriented and is missing somewhere inside the fire unit. This is a signal to the other firefighters so that they can initiate a search.
"Standby for the Hotshot...."