It just gives me prompt like this
talk to every person you see roaming the street or standing there. some give you a clue but at some point most of them simply tell you about the sex virus. My last clue (maybe 2 or 3 of those actually mentioned it) was where to find my last chest.
1st picture, i don't really know but i guess it's something you get done when following the story or ! points on your map.
2nd picture: go to the apartments and there should be half a key on them somewhere
3rd picture: could refer to the chest hidden in the mansion, if you head to the dining room you can enter the wall directly below the table, open a vent and get to the lower section where a chest is located.
4th picture: if i remember correctly there's a red switch inside the office of kimura where you need to use your electromagnet, it should move an orange traffic cone in the police station so you can enter the wall there.
the whole sex virus gossip i believe is simply something they say when you found everything you can, once there's a new update they probably going to have new riddles to solve.