@Chaz Hoosier, to be clear, not ignoring you, but Morn is in the enclosure with the oak and the others, while the new arrivals are at the entry to the ruined keep and cannot see him or be seen by him from that location.
QuoteHowever, when the tiny rogue detects an Elf approaching the oak tree room and then hears a second one speak
These elves have not yet approached the "oak tree room", per the map, they are at the mouth of the ruined keep. Jesse can spot them from her high perch however.