I have stored my AR in a closet on a rack.  I recently discovered a leak from the outside sprinkler system has soaked and mildewed the carpet in the closet.  I looked over the rifle for rust and did not find any.  I live in Las Vegas so it is usually extremely dry here.  The closet however surely was more humid.  This got me thinking, what parts of an AR are most succpetable to rust?  The rifle is a RRA M4gery W/o a chrome lined barrel.  

I'd run a brush, dripping with CLP through the bore, then follow with a patch..You'll know if the bore has rusted..Look under the handguards as well..

Basically anything that a magnet will stick to on your AR has the potential to rust, though a bit of CLP will all but eliminate this.

Meplat-

First place I have seen rust on folks ars is the inconspicuous areas that don't get oiled by your average, casual shooter.  Threaded pins for removable carry handle, grip bolt head, ect...