Person leaving early may vote remotely

A person who attends the beginning of a meeting and is issued a code card may leave the meeting and keep the card, still knowing the URL and agenda index.

Solution proposal:
Provide a second factor of auth - a common token which is replaced for each vote and only displayed in the meeting room. By revealing that token only when the vote takes place, each voter must be present or have a rogue accomplice present to transmit the code.