Nice of you to pop in David, been a long time. Welcome back.
Can you post a picture of a typical basement? Usually a home with a basement has a hung sewer and an ejector pump for fixtures located below the sanitary sewer. The ejector pump will have a check valve and that’s it. I’d like to see the plumbing system with these check valves. Are they on every fixture?
Not all waste piping in hung in Massachusetts. Direct route out to street . No, rarely will I find a check valve.
How we doing buddy. I haven’t been in the MB in years as my life of work, kids, and wife keep me busy every day.
I hope all is well with you and yours
That’s a hung sewer. A hung sewer is a sewer that exits the basement above the basement floor.
Up here, like mine is all under the floor with a cleanout and floor drain just before exit if you are lucky and if you have a private system with a basement, they all exit above the floor. No check valve.
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That’s way it usually is 
A back water valve would prevent this. Back water valves should be installed downstream.
Source? Is this just your opinion? Plumbing is often installed with wyes on their side…sheesh.
Back water valves are installed on a sewer main NEVER a drain branch.
Example below from IPC
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Every fixture trap is required to be vented as prescribed by one of the methods in chapter 9 of IPC. Which method is being used in the OP’s pic? it cannot be the stack, if that is what you were referring to, because the stack must be vertical when used as a vent for the fixture. Let me know 
This shower drain is vented properly if a circuit vent is utilized per chapter 9 of the IPC🙂. I see circuit vents used all the time over here. You’ll never see this method in UPC regions. It’s a good question Simon as this method of plumbing is very difficult for me to accept. I just did a construction inspection where a branch drain rolled up on a circuit vent over a HVAC duct. The p trap is now a s trap as far as I’m concerned. I requested a re-vent on the drain and have yet to hear back from the builder. Del Webb boo!
You stated it does not need one… now you are saying it’s circuit vented? how do you know this from the one pic posted? the circuit vent is a special type of venting, rules must be followed. The vent/drain pipe of a circuit vent cannot exceed 1" per foot slope and must be uniform. It also has to be a branch. What’s pictured has a vertical offset. That alone makes it wrong.
Are you certain what is pictured is kosher?
Where did I say that? It does not need a separate vent for the shower if this particular fixture is located between the circuit vents.
Only a small portion of the drain system is visible. What I am telling you is if this is part of a circuit vent or a wet vent no separate vent is required. It’s pretty simple to understand.
Looks good from what I can see.
Pitch and grade is uniform. What has to be a branch? Simon I will include a picture of a circuit vent. Study it and tell me how is the OP’s picture a violation. Are you assuming the 3” pipe upstream of the shower is a toilet?
This is where you said it. I asked about “a vent”, not about individual/common/separate vent. What is in the pic looks wrong to me and I wanted to see how it was being vented (whichever the method).
You then followed up saying it could be a circuit vent… I said okay, but if the fixture (shower) is being circuit vented, the circuit vent’s main horizontal branch, in essence a wet vent, cannot have a vertical offset. You showed me some pic to study, well I studied it good Does your pic show a vertical offset we see in OP’s pic? no it does not.
Now… while IPC is not specific on the offset, the following implies that the wet vent of the circuit vent cannot have vertical offsets:
914.3 Slope and Size of Horizontal Branch
The [slope] of the vent section of the [horizontal branch drain] shall be not greater than one unit vertical in 12 units horizontal (8.3-percent [slope] The entire length of the vent section of the [horizontal branch drain] shall be sized for the total [drainage] discharge to the [branch].
If you are not convinced, maybe this explanation from the friends at UPC (the code you much prefer to IPC) will convince you. BTW, UPC also does not specifically reference an offset but they do imply it via the max slope. If you disagree with this, you’re not explaining why Lynne Simnick is wrong and you are correct:
I’m not assuming anything, I cannot see how the fixture in OP’s pic is vented, period. If it’s an attempt at a circuit vent (which I doubt very much), it’s wrong because it does not follow proper principles of circuit venting. I will bet they just tapped into main drain.
