We do Whole Genome Sequencing, and sometimes we outsource the sequencing. We always get the excess of DNA back, and it is stored in our own freezers. Even in this scenario we can't be 100% sure they don't store the DNA or the files for their own purposes, but that's the risk we assume. The DNA we send is only identified by a number.
I can 100% imagine a company such as 23andMe storing DNA for later sequencing, or even doing WGS to do their side business, while sending you back only the genotype. Did you request your excess of DNA back? No, you didn't, because you didn't even know how much you sent or how much is needed for a genotyping. What you did was linking your DNA with your real name and some extra data, so further data augmenting is trivial.