Trivia: The film was conceived with Macaulay Culkin in mind to reprise his role as Kevin. Culkin refused, however, thinking that a third adventure with the character would be too unbelievable, and also feeling a bit burnt out with the role. Thus, the film was rewritten to focus on a new lead.
Trivia: At one point in time, the movie was going to be made for television and serve as a backdoor pilot for a potential "Home Alone" TV series. But this idea was dropped for a traditional theatrical film. The following movie, "Home Alone 4: Taking Back the House," was written to be a stand-alone movie but was retooled to serve as a backdoor pilot for a potential TV series. (Which of course never happened).
Trivia: One of the many early concepts for the film after Macaulay Culkin dropped out was to have Joe Pesci and Daniel Stern returning as the "Wet Bandits," who would torment Kevin's young cousin Fuller, but this was dropped as none of the actors wanted to return for a third film.
Answer: It was Jernigan who thought there was a woman in the shower and he couldn't take the risk that if it was a woman, that she would eventually get out and see intruders in the house and call for the police. The only option open to Jernigan would be to kill her so there'd be no witnesses.
No it was Beaupre who thought there was a woman in the house.
I've seen the movie and I can confirm that it was Jernigan who opened the shower curtain because he thought a woman was in the shower.