Field trips provide the perfect opportunity for students to get lost, as they are usually in an unfamiliar setting which is often open to the public. In order to quickly find if anyone is missing, have your students organized in groups of 3. When you check for attendance, simply have everyone make sure that their teammates are present. Also have the groups stay together throughout the trip. You can also assign each group a number and then just call for these numbers and look for 3 hands. This will help save you time and you won't worry about students getting lost.