This has happened to me many times. Last night in fact!
So had to revisit a project that was backed up.
Restored it to my RAID and opened it up. The Event containing all the projects had the alert sign.
Weird as the Library is self contained.
Trying to relink showed no clips available as everything according to FCP X was online.
So knowing I had a back up I thought I'd start deleting projects until the triangle went away.
Lassoed a bunch and deleted them. BOOM. Triangle gone.
Hit UNDO and the projects came back but the triangle did not!
So my guess is that something in the event file was refreshed.
I must admit I really hate the obscurity of the warning sign. Would much rather it live on the project itself or at least give me SOME kind of info as to what it thinks it's missing.
Sean Lander - Editing since 1982 - AVID 1991 - FCP 1999 - FCPX 2011
Ditto re : mysterious warning signs.. Sometimes there seems to be no way to find out what the issue is ( I've followed all the advice above). Surely FCPX should give you some help as to what the issue is!??!