How do I find and dispose of the dead animal that's presumably in my ceiling?
Oi! this reminds me of an experience I had about seven years ago.  Horrible smell with no obvious source.  Turned out that the cats had killed a rat and hidden it behind the books on the bookshelf.  Only noticed it because of an awesome slimy trail of maggots.

Anecdote aside, my recommendation with this particular occurrence is to wait it out: rodents tend to decompose in a relatively short period of time, and the smell will dissipate within another week.  If it persists you're dealing with a larger animal - say a full grown opossum or bigger - which leads to the really pertinent issue:  how did the offending critter get to where it got before it died?

As a self-respecting homeowner you should do whatever you can to prevent entry by pests into your home.  Sometimes this involves a thorough investigation by a professional but for the most part an observant individual can determine likely points of entry and fix them.

Most modern houses are covered by siding/doors/windows in every location with three major exceptions:  foundation line, crawlspace venting, and eaves.  All apertures can be sealed with some combination of welded wire, steel wool (coarse) and possibly adhesive by a determined homeowner.  The more aesthetically pleasing approaches are less simple and more costly, but there are lots of solutions out there.

As far as your storeroom goes - I recommend rocking->taping->mudding to close any access points for the critters, same with any unfinished space that is prone to pest invasion.

Hope you have a better olfactory experience in the near future!