Dahmer's victims were young gay men, who would be disregarded by society, but on the other hand he killed 8 people in 1991 alone. It seems like that might have at least caused noticed in the community that 8 people had gone missing. Was there any inkling either by police or the Milwaukee gay community that there was a pattern of young men disappearing?
Dahmer's first victim, Steven Hicks, was killed in 1978. His parents filed a missing person's report and there was an investigation at the time, but it was eventually classified dormant.
The family of Richard Guerrero hired a private investigator after his 1988 disappearance, because they felt the police were not taking the case seriously - most likely because Guerrero was Hispanic.
The mother of 14-year-old Jamie Doxtator reported him missing in 1988 as well, seemingly to no avail.
Anthony Sears's mother did not report him missing right away, because he often left for a while with friends. According to the below FBI file, though, a friend reported him missing within days of his disappearance.
According to this FBI file (FOIA request, PDF), Oliver Lacy, Jeremiah Weinberger, Anthony Hughes, Konerak Sinthasomphone, Errol Lindsay, Ernest Miller, David C. Thomas, and Edward Smith were all reported missing. Matt Turner, Joseph Bradehoft, Raymond Smith, and Curtis Straughter were not.
So this answers your question regarding whether investigations were opened into disappearances - yes, absolutely. Were they effective, though? Was a pattern noticed? Milwaukee's main gay news publication at the time, In Step, devoted almost an entire issue to it (PDF link) in the wake of Dahmer's arrest. Of particular note is this statement from page 8 of that issue:
Police in Milwaukee and Chicago had files on missing persons. Many of them were younger, Black Gay males. Why didn't anyone put all the facts together before it was too late? What efforts, if any, were coordinated by the police or others? Most of Dahmer's victims had a common sexual preference, were Black, or of a lower economic status. Those should have been clues that something was amiss.
Why wasn't the community made aware of all these similarities? Why weren't we warned we had a 'boogeyman' stalking our community? Is is [sic] because the police don't pay as much attention to problems affecting the Gay, or Black, or poor? Is society so bigoted and homophobic that our people aren't deserving of help on the same level that would have been exerted if these had been straight white women that were disappearing left and right?
It definitely sounds like the police didn't notice a pattern, or if they did, they didn't change their actions based on this. Without paging through many years of a weekly magazine, I can't say for sure if In Step ever reported on community concerns, but the above paragraphs make it seem like the Milwaukee gay community had no idea. The rest of the magazine is a pretty scathing indictment of the Milwaukee police - well worth a read.