First, I sadly couldn't reproduce it, nor I have observed this live, so this is consideren an incomplete report and any help on reproducing this is appreciated.
However, I got a depot in Lytingcombe that was only built to start a mail line. There are no passenger routes around, at least none served by me.
In that Depot were 11 of my old Friesian/Stagecoach convoys stored which, according to their set route, belong to a Rose Yoxchurch-St.Clement line on the other side of the map.
I asume these could not find a depot when they should get replaced and something went wrong in detecting the "next" depot to teleport them, but as I said, I couln't reproduce it so far.
Did anyone else observe such behavior, or even better could reproduce it?