Could be rats. My friend experienced the same thing, rats in her birdseed in the shed. I had rat(s) eating the dog food from an opening under the house into the cupboard - YIKES!!. We've since repaired the hole and killed at least 4 rats in traps. And we have hawks and owls around here (near Decatur) -- and I have apparently USELESS terriers! What can be happening?
rb

I use a metal garbage can to store grass seeds and bird seeds in. Sure keeps those guys out. I once had an infestation of rats from allowing them to get at a 50 lb bag of grass seed. I eventually killed over 20 with traps before they were all gone. Now the cats keep everyone in control and I even put a cat door into the greenhouse so they can hunt in there also.

Sounds like rats to me too. We have them in our shed at church. BLECH!
Store your seed in metal, and if you use organic soil additives, those need to go into metal containers too. Do you have a compost heap?
If so, stop putting food waste in it. THat discourages the little beasties from finding food and a nesting place close by.
You are just going to have to bait, or let your cat in there. At church we set out both chew bars and traps.
It sounds disgusting but I leave the dead rats in the traps for a bit. Their presence is a reminder to other rats to KEEP OUT.

GGG

Okay so now you have me worried. We have had rats eating from our birdfeeder. No problems in the house although they did build a nest in the grill once. I am not sure who was more scared when DH lit the grill after not using it for a few weeks, the slightly singed guys who ran out or my husband who witnessed it. We had to stop leaving dogfood in the dog's bowl on the porch (inside dog, but she likes to eat outside sometimes) because they were hiding it in the grill all the time, (ever smell burnt dogfood? Nasty) but this time they had actually built a nest under the grill grate. Since we stopped feeding the dog outside they seem happy enough to munch from the birdfeeder and they haven't caused any damage, am I being naive to think I can just let them be? I really hate to kill any animal if he is not doing any real harm. I figure as long as there is free birdseed they aren't going to cause any trouble right?

rat traps are set. have to see what happens next

If you are feeding rats, chances are very good that they will try to nest close by. Rats like to feed and nest in short distances.

well, I have not seen nor heard any animals down there. The traps have not been set off and everything is still clean. Now I am wondering where the critters may have gone!

Have you checked the birdfeeders? LOL