Technically speaking, Janelle and Mike broke into 1717 East Delmar. True, my door would never be left unlocked. However, if my friends noticed my car at the house and decided to let themselves in because I didn't answer the door, I wouldn't be very happy.
Whenever I was 7 and under I would walk into Great Aunts and Uncles properties without knocking. This is because I was a child who lived a "stone throw" distance from them at the time and had their permission.
What do we know about Sherrill? We know she took pride in her home, enjoyed working on projects, and was somewhat of a "private" person. Needless to say, I don't think she would have been happy coming back to learn so many people rummaged throughout her house while she wasn't there. IMO someone who came by Delmar that Sunday knew Sherill would not be returning.
I also agree with the whole "sex could have happened in the house." In High School, I had a pair of friends who didn't care where they had to sneak off to do the deed. Also. from all the movies and shows I've watched filmed in the early 90's sex was much more of a "big deal" back then. Almost everyone was doing it, but parents still wanted to cling to the idea that "abstinence" was what all the kids were practicing.
Speaking of it being the 90's, manners also played a heavier role at the time. IMO it would be very rude to enter someone's property and rummage through their things.