Not sure about the US but this is more or less what happens in Japan: Brokerages are obligated to flag and report suspicious orders or trades to the [equivale

Not sure about the US but this is more or less what happens in Japan: Brokerages are obligated to flag and report suspicious orders or trades to the [equivalent of] SEC. If necessary, the SEC then asks the brokerage for more information and investigates it. The definitions of "suspicious" can be somewhat arbitrary, but those >1000% returns would definitely be considered suspicious. Two people at the same workplace getting abnormally high returns would also be considered a red flag.