I've never tried to store them myself, but one of the books that I got when I started gardening was the "Encyclopedia of Country Living", which is loaded with information on growing, canning, storing, etc. Its recommendations for globe onions is a follows (minus the typos of my transcribing):
Curing Bulbs. Let them sun-cure for 3 days. Spread them out on top of the ground until tops are thoroughly dry. Then sort. Bulbs that addicentially got cut with the space go into the house to be used soon. Sets that for some reason didn't take off are cured some more (maybe 2 weeks in a shady, airy place) and then saved to be given another chance next year. They often make it the second time they are planted. The winter storage onions are cured some more and then bagged for keeping.
Storing. If properly cured and stored in a dry, airy place, many of your winter-keeper onions will last 4 or even 6 months. Every summer we bring in onions from sets by the gunnysackful for our winter supply. The trick is to sure them very well in the sun for 3 days or so before bringing in. Rip off the "set" portion before drying if you can. The onion grown from a set has 2 parts. One is small and extends up into the a stiff hollow stem. That is the part you should rip off -- or else the onions won't keep long.
After you bring your onions in from the field, continue drying them. We have tried all sorts of systems for this. You can braid them by the dry stems and hang them up in bunches with wire or twine around the bunch. Or them them in an "onion"-type bag or into old pantyhose and hang them up. My current method, and easiest one yet, is to dump them into cardboard boxes and bring them into the kitchen. I check them occassionally by running my hands through the boxes and removing any damp ones. Any onion that feels the least bit damp is on the verge of spoiling. When the rest are absolutely bone-dry from the kitchen's heat (once that old wood cookstove starts going, it really gives them the treatement), they can go under the bed or into the attic -- anywhere they won't freeze and will stay dry.
Slight freezing doesn't hurt them, provided you don't handle them while frozen. Good ventilation is very important to their storage place, and what's why they should be in some sort of loosely woven bag or hanging basket.or braided and hung up or some such, although I get away with a box. They do better stored dry and cool than dry and hot because with too much heat, they have a tendancy to gradually dry away to layer upon layer of papery nothingness as winter goes by.
When we use our big winter onions, we take the largest first and gradually work our way down in size. That way, if there are any left over come spring, they will be small ones suitable to use as sets in the spring planting.
I should also note that I've seen the pantyhose method in use before, I think at my godparent's place. They had loaded an onion into the toe of the stocking, tied a knot, loaded in the next one, tied a knot, etc, so the onions didn't actually touch which might affect their drying out, and then hung them up so they weren't touching anything.
The smell is of rancid fats in the flour. It is a very distinct smell, but hard to describe. I would agree that "sour" is part of it. The best way to learn is by comparison. Smell some fresh, clean flour and remember what it is like. Any kind of unpleasant odor than that is rancidity. You may have smelled it also if you opened a box of cereal that has been around too long, or even poppy or sesame seeds - anything with fats that sits around at room temp will eventually go rancid.
Best Answer
The reason flour is in paper bag (either 1kg/2lbs bags from supermarkets, or 25kg for bakeries) is to let it "breath": to get it oxidized. If you see an old (vintage) bag it's made of a net that lets a lot of air to get in. Today those bags are not used because it also allows bugs to get in.
When wheat grains are just milled, the flour is not good enough: it needs about 2 weeks of storage to get aged. But once aged, you don't need paper bags anymore.
Wheat grains have oils/fats inside. After 3 months of storage there will be too much rancid (oxidized, ¿stale?) oils in the flour and the taste won't be optimal. Trying to keep it in hermetic containers won't prolonge too much that time, as there's too much air in the flour.
Of course, modern industrial millers won't wait 2 weeks storing a product they could sell earlier: they add enzymes and chemicals to accelerate the process. As well as they'll add chemicals to postpone the "best before" date.
After that date, the flour will taste more rancid, and its gluten won't work as well. To delay that rancid tasting, you might try to refrigerate the flour (IE: put it in the fridge) but 25kg are too much for a fridge. If you don't notice the taste at all, there's no problem eating it. The gluten won't work as well as when flour was "young", due to an excess of enzymatic activity. You'll notice this effect as the loaves won't rise as well, and the crumb will be denser and chewier. This could be solved adding very strong flour or wheat vital gluten (or just enjoying a denser bread).
Another problem you could get with long storage is some bugs appearing in your flour. There's no way to prevent them: their eggs are in the flour when you get it. But they usually appear only when it's hot, so you probably won't get them the next months (if you live in the northern hemisphere). In the summer that's one reason for me to convince my couple to store my flour in the fridge. An "old, wise trick" I was told was putting a leaf of laurel in the flour to avoid them. It never worked for me.