Hi, I got a bit lost with this subject and was tired of reading posts about people who doesnt really know what is going on about the Navigation and they trial/error to take this Red Flag out of the game. There are multiple causes you got the Nav Mesh Need to be Rebuild flag, but most cases its about the level nesting. Just be sure the NavMesh Recast is in the main level and the blocking volumes and navigation volumes are ALSO in the main level. And it´s gone. Hope it´s of help for anybody, sorry about my english!