From the Globe article:
One member of the team confirmed to the Globe that there was constant traffic in 2018 to and from the area where the real-time feed used for determining whether to challenge on-field calls was broadcast. However, that source noted that the real-time video replay system was located next to the BATS video consoles that players use to review in-game at-bats.
According to that team member, Red Sox player traffic around those systems was constant — in no small part because players were constantly reviewing their swings and pitch selections from previous at-bats. Indeed, that same team member noted that it could have been possible for sign sequences to be stolen via the BATS system, albeit in a fashion that was slightly (at least one batter) behind real time.
...the proximity of the BATS system to the real-time feed also would have created what was characterized as an almost unavoidable temptation to crack teams’ sign sequences, something that could be used to relay pitch types and/or locations via traditional means.