Under Tresendar Manor,
Room 4 of the Redbrand hideout (on p. 22 of the adventure), where the Tresendar crypts are, contains 3 animated skeletons. They attack any creature that approaches either adjoining door unless that creature wears the Redbrands' cloak or speaks the password.
It looks like the animate dead spell only works for 24 hours at a time. However,
Glasstaff (using the evil mage statblock on p. 57) isn't nearly high enough level to cast it, and doesn't have the spell in his statblock. Also, the skeletons are not "free-roaming"; they are working for the Redbrands.
Who made the skeletal guards in Tresendar Manor, and how were they constructed?