I've spent time around more babies & toddlers than most people see in a lifetime (sometimes >50 per day; probably no more than 2000 per year though; for >decade; [working at baby & toddler groups]).

Could you say if this was a fixed behaviour (ie they always sought a private place to poop), had a fixed location (eg always behind the couch), if you used a potty at the time, and exactly which month it appeared at so I can note it with my own observations. Cheers.