I own a street sweeper company, and avidly metal detect. I used to keep my eyes peeled as we dumped our sweepers. Yes it's true they can pick up coins. And of course, paper money. But it certainly wouldn't be worth your time sorting through the putrid trash, just for that goal, in my opinion.
And the type sweepers that run around parking lots at night, to sweep up litter, are not very strong. I mean, if they are really tuned tight (no air gaps in the sweep head, no holes in the tank to lower air pressure, etc...) then yes, they'll pick up pebbles, coins, etc... But more likely, they are out-of-tune and fly along fast, because they really only care about paper and litter, not grit, nuts, bolts, etc.... That industry is really competitive, so if you're going slow to make sure you don't miss dense items, odds are you'll loose out on the bid. That's why we got out of that line and work, and are strictly the big municipal & construction-related type sweepers now.
As far as municipal street sweeping (the kind that go in front of your house on city streets public works sweeping), it's going to be the same thing: sure, they'll pick up valuables now and then, but I simply can't imagine it being worth someone's while for clad, for pete's sake. Now maybe if you knew where the sweepers from the 1950s/60s dumped out their load, and then you might be talking silver. But for clad, the relative ratio of trash you'd have to be hearing, wouldn't be worth your time, IMHO.
