It's happened to me when my hotspot phone would die at start of trip and I wouldn't notice until I would try to end the trip.
When I drove Uber I got into the habit of going back and looking at all my recent trips every so often to make sure I catch it anytime it would happen.
Now that I only drive Lyft I still go back through my Lyft trips and look for trips where the $ dollar amount is less than the miles. This can be done from the list of trips without going into each one.
Fortunately on Lyft it tends to record your GPS even if you lose connection to the server and it'll let you end trips even when you're offline. When it regains connection it uploads everything. I dont think Uber does this but not sure.
Check your history. You always gonna find a few extra bucks..