Format does not remove data. Not even a full format. Hope you didn't have anything you are concerned about on the drive. Most people are not going to look for data on a drive they think is blank anyway. Blame MS for their crappy implementation of formatting.
Easiest way to erase the data without external programs, is to copy files to the drive you don't care if anyone sees , till the drive is full, then delete them. If they try to recover the drive all they will get is the last files you put on there.