Looking at some old google articles, I found a few odd things:
1) A very large search group that also included 500 Boy Scouts combed Highland Park after (2) leads. One was a child finding a bracelet near the park entrance. Mary Ann's Aunt and three of her friends said they could identify this as hers. This was later discounted, as they noted that many girls had similar bracelets (faux pearl w a heart charm) at that time- and investigators concluded she could have lost it there at some other time or it could have belonged to any young girl. However, the day after she disappeared an 8-year old child reported seeing a girl that fit her description running and screaming as she ran, towards a wooded slope. Another article said the child was screaming for help. Both the bracelet and the young child running were in proximity to either the Children's Zoo section OR a service entrance in that area. These events prompted the large, extensive search.
2) A man reported seeing her outside the Martinique trying the door, which he said appeared to be locked.
3) In 1958, there was a murder at the Martinique Apartments- Mary Regan, age 45, found by husband. The husband seemed to have been cleared and I can't find any other reports that the case was ever solved.
4) The janitor is listed as a witness in seeing Mary Ann that day, in the hall. He is also listed as seeing Mr.Regan around time that wife was discovered deceased.