The league told video monitors working at Minute Maid Park to listen for banging sounds coming from the home team's dugout early in the 2019 campaign, according to Chandler Rome of the Houston Chronicle.
"What they told us was we were essentially looking for people who were using technology to steal signs," said one video monitor.
Meanwhile, a video monitor that worked in another ballpark said they were not told to listen to any sounds from the dugouts, according to Rome.
The league reportedly started an extensive investigation after right-hander Mike Fiers revealed the Astros set up a camera in center field to watch opposing catchers in 2017, and then had players or employees make loud noises to indicate to hitters what pitches were coming.
An Astros executive also reportedly asked scouts to steal signs by using cameras in the same year.
It hasn't been confirmed whether Houston used the system during 2018 and 2019.