Typhoon Call the police, tell them what you see and let that be the end of it, you did your job now let them do theirs.
This is what I did with my next-door neighbors. Two young 20 something kids moved into the house and something didn't add up. Very affluent appearance, both very attractive and in a committed relationship but not married, drove BMWs 3 series but apparently didn't work and he said they were going to attend the nearby university. He said he played golf and asked if I'd take him to my club sometime and introduce me to my friends as he was considering a membership somewhere. He always had an excuse when I did invite him to play on Saturday mornings with my group. I had my realtor check on the purchase price and any details - he calls back and said the house was now owned by a trust. hummmmmm
Then the cars started arriving for quick in and outs of the house. I have young neighborhood children and my kids playing in the yard all the time. I'm thinking all I need is a bad deal to go down and I am genuinely concerned.
I called the non-emergency police number and asked to speak with an officer that works on drug cases. I gave him the basic info on what I've observed and my concerns. I also said there are local city pick-up trucks that stop by there during noon hours when I stop by to check on my girls and their sitter and that got the detectives attention. He thanked me and told me they will put the house under surveillance and he said their way of doing this is putting up a fake electrical transmission box that contains a video camera on the nearest power pole. A couple of days later there's the actual power company truck out there putting up a grey can.
It wasn't much later that the traffic stopped coming by and within 2 months the couple moved out. It's been many years ago so I don't remember details but I did start getting calls that immediately hung-up from unknown numbers. Maybe the couple was trying to harass me but soon that stopped too. I remember finding out that the couple both had trusts they were living on and had been in drug rehab at the Menninger Clinic then headquartered in Topeka and released to assimilate back into the community. I felt like I helped to take out the trash in the neighborhood.
Today I would have used our anonymous Crime Stoppers hotline. They are successful at protecting the tipsters identity and even offer rewards for successful prosecution. I'd check to see if that is available in your city/county.