Need Help with plumbing problem, please. House has a septic system & doesn't have p-traps. Is this right?

I had this home inspected by a home inspector and he did not mention anything about the lack of p-traps. I have not had a home that has a septic system before so I thought that the no p-trap thing was probably OK, but did not know. Occasionally we have sewer whiffs of smell from drains and now only the hall... show more I had this home inspected by a home inspector and he did not mention anything about the lack of p-traps. I have not had a home that has a septic system before so I thought that the no p-trap thing was probably OK, but did not know. Occasionally we have sewer whiffs of smell from drains and now only the hall bathroom has a mold/mildew smell coming out of the sink and tub drains. The tub has an overflow and the sink does not have an overflow. The master bath set-up is the same as the hall bath, the basement bathroom sink has no p-trap and no overflow. There is a toilet in that bath also. If I go out in the garage I can see a p-trap from under the master bedroom bath area tub. The roof vent is also right between the hall and master bedroom. There is not a roof vent by the kitchen area (15 or more feet from baths) and that is where we get the sewer whiffs (not very often). Do I need to put in p-traps? Is the one roof vent OK? Any cause for mold smell? I have checked for leaks from tubs and sinks etc. Please help!
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