Toilet overflowed and water got under the laminate. Mold?

[Cali Skier]

So I just bought a house, this is the 2nd month I have rented it. The renter called last night and said that the house was flooding, toilet in the master and AC drain in the garage both overflowed.

I called the former owner and he said remove the cap off the cleanout outside and plunge it. The renter did that and it cleared. I asked the owner if this happens often and he said it didn't but this is the fix when it does. He said if someone flushes something they are not supposed to it will cause this. Sounds to me like it is clogging beyond the cleanout, odd...  The former owner said something about a "flap" that gets "stuck" or "clogged" not sure what that is about.  This is a line that goes to a city sewer line that runs along the back fence in the neighbors yard.  

Questions:
1. I have never seen this happen on 4 other houses I have owned. Is this a common thing? Does something need to be done, or just tell them to be on the lookout, try to watch what gets flushed, and plunge it if it clogs again?

2. The water got under the laminate in the master. I think I will shop vac it heavily tonight when I go over there. If no other water is going to be present in the future should I just do that and leave it? My thinking is there will be no further water for mold to grow. Any thoughts on this?

3. The "flap" anyone know what that could be?
Thanks in advance folks...