hptSomeThingsBelowUs missing module

[Evan Laforge]

Ever since upgrading to 6.12.1, I've been getting a new warning.  I
compile with --make, e.g. (clipping out irrelevant stuff):

ghc-6.12.1 -threaded -W -debug   -package ghc --make \
		-main-is App.Main App/Main.hs \
		-o build/seq

If I remove all imports to a module (say Util.Debug), compiling other
modules prints this warning:

WARNING in hptSomeThingsBelowUs
    missing module Util.Debug
    Probable cause: out-of-date interface files

I searched on trac but didn't see anything with this name.  Is this a
known issue?  It doesn't seem to cause any harm in the eventual
binary, but is somewhat alarming to see a bunch of them scroll past.
Manually wiping out a bunch of .hi files causes recompilation and the
warning goes away.