This was the first chapter of what turned into Spygate. Stepney was dismissed from Ferrari under allegations of sabotage, before the espionage details started to come in. The precise details of the sabotage accusations were not made public, but this is from a news report at the time:
Stepney says he is innocent of allegations that he tried to sabotage the Ferrari F1 cars in the days before the Monaco Grand Prix … Ferrari refuses to give details about the allegations it is making but someone somewhere is feeding the Italian media with stories about white powders being found in the fuel tanks of the Ferrari racing cars in the run-up to the Monaco Grand Prix.
The same day, McLaren suspended a senior member of their team after they learned of Ferrari’s internal investigation, but did not reveal who it was (we now know it to be Mike Coughlan). And the rest was history….