This was in reference to the 2014 sales of Berkshire's position in the British supermarket company Tesco.
It was not Warren's first use of the metaphor.
In 2007 (pre-collapse/conservatorship!) he was testifying in the government's case against Freddie Mac's CEO and asked why Berkshire had bailed on a long-held position in the stock. Buffet mentioned earnings smoothing as the tell and said:
"I follow the old dictum: There's never just one cockroach in the kitchen,"