This seed is called "COIN HEIST"
The kids are going to the local video games arcade as usual and wasting their pocket money (something I can say as an adult but, hey, I did it too). They notice a guy in the arcade they've not seen before. He holds a strange wooden box with two lights and a switch up to the side of one of the slots and then leaves, sniggering to himself. Honest kids will report this activity but there doesn't seem to be anything wrong and Miko, the arcade maintenance guy, berates them for wasting his time.
Time passes. Play another scenario.
Mike calls them over and asks them if they have seen that guy again. Turns out there's something wrong with a load of the machines, takings are way down. Miko tells them that if this keeps up, the arcade will be forced to close. The kids now have a mystery and one with the heart of their childhood at risk!
What's happening is that the guy, an impoverished quantum scientist at The Loop, has invented a spatial vortex machine. When held up against a certain model of slot machine and activated, it will steal all of the coins of a certain size or shape that enter it and teleport them to who knows where. The guy, Halle Sikorsk, has high debts but isn't entirely stupid. He didn't want to steal all of the coins, just the cupronickel-clad nickel 5 kronor coins. Silver and cupronickel-clad copper coins were not affected.
They may spot him in town but most obviously because while he's insanely smart, he's also insanely dumb. Rather than pay off his debts, he's bought a brand new 1984 Porsche 944. They may also track him down by asking at banks if people have been depositing large numbers of coins. He's living in a 3 bedroom house and in one of the rooms is a box attached to the ceiling, out of the bottom of the box fall 5 kronor coins in a constant stream, into a complex series of chutes. There are coins everywhere, overflowing out of barrels and bags. It's a kings ransom. The machine is fragile and the effect wears off and has to be renewed. Halle needs to go to jail. He could be trapped by putting marked coins into the machines and then getting the police to search. That's a difficulty in itself.