How effectively can a trancing Elf serve night watch?


Experienced DM, new to 5e. Dealing with a party that includes two Elves. Party did not establish a night watch. Dice dictated an overnight raid of the party’s provisions. Seems fair to give the Elves a chance of hearing the activity, although being situated 50′ away, it would not be great. However, having two of them should boost the math a little.

What is fair to do here, given 5e rules?