Not true. It’s not uncommon for a toddler to find a private place to defecate without being told. There is an innate sense of a need for privacy for some things.
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.