In the first book the group is focused on getting control of the palace and is not worried about the return of the army because they think that if they get the palace they'll have the atium and, thus, the ability to pay for the loyalty of the army, since they're mercenaries. Well they didn't find the atium, right? So no ability to pay the old army for their loyalty. I'm just saying that it seems to me that the absence of the Lord Ruler's army supporting the city should be a huge indication to all the other warlords that the atium isn't in Elend's possession.
People have been saying that he wouldn't have anyone to trade with so the atium would be worthless but he would have had a year to work out trade with someone if he wanted to monetize it (with as valuable as atium is made out to be in the first book he would have found someone wanting to buy some even if he couldn't get the price the Lord Ruler charged for it).
Ah! Well, that makes sense. I certainly think some people in world might have come to this conclusion. But they'd still think the atium must be somewhere in the city, even if Elend doesn't have/isn't spending it.
However, I think the issue is a little less cut and dry. For one thing, Elend DOES have the loyalty of much of the local army--the garrison serves him now, which would be an indication that he has access to some of the Lord Ruler's resources. I don't think the lack of a larger army would be an indication he doesn't have the atium, however.
Let's say you were a small band, and were able to seize and control Ft Knox, and get the guards stationed there to serve you. The president is dead, and the country is fragmenting into city-states.
A senator from a nearby state might easily round up the military in his area, promise them that he's the new civil authority--then push them to go seize the gold. When they arrived, they wouldn't think, "Why is their force so small? They must not actually have the gold." They'd think, "They're ripe for the picking. We got here first."
That's basically what is happening. The "Lord Ruler's Army" doesn't exist any more--it's fragmented, taken over by various groups who ruled their own local regions. And the bulk of the most frightening part, the koloss, are their own uncertain band.