“Mysterious Algorithm Was 4% of Trading Activity Last Week”

According to CNBC an unknown program triggered 4% of trading activity last week:

The program placed orders in 25-millisecond bursts involving about 500 stocks, according to Nanex, a market data firm. The algorithm never executed a single trade, and it abruptly ended at about 10:30 a.m. Friday.

Someone testing in production perhaps?