Various npc theft related questions

How does npc theft work? My base is on an island near impossible to get to by swimming. Thieves that stole anything would likely need a boat or be unable to leave with much due to the weight of the items. I have random npcs enabled at default frequency. Can thieves spawn on the island and rob me? Will they despawn afterwards or remain on the island? Could they use a boat to reach me (afiak npcs cannot drive)?