mds and mds_stores constantly consuming cpu mds and mds_stores are constant CPU consumers even when I think nothing should have changed on the hard drive.
Activity Monitor:

I have a circa 2009 Macbook Pro with lots of software installed over the years. Dropbox and Crashplan are active background processes among many others.
I have included many locations in the privacy settings of Spotlight to exclude from its index:

USB drive
Crashplan folder
/Library/Caches
/Users/me/Library/Caches

Still, mds and mds_stores are constant consumers of CPU hovering around 10-12% each. My fan keeps running due to the heat.
Any ideas as to what it could be due and how I can troubleshoot?
As you know, the mds and mds_stores are Spotlight activities.

The reason why your Spotlight is so active could be a number of things; it could be you have an app or multiple apps constantly changing some folder contents.

First let's check whether Spotlight is the cause of the fans running so much. To test this, run the following in your terminal:

sudo mdutil -a -i off


this turns off indexing of files, and should result in a clear slow down of the fans if mds and/or mds_stores are to blame.

To turn indexing back on, run 

sudo mdutil -a -i on


After this you could run the complete re-indexing of your hard drive (be aware this could be an over night job), it will delete your Spotlight data base forcing it to start over.

sudo rm -rf /.Spotlight-V100/*


The next and final step would be to add others to your (do not scan), privacy settings.