In rural areas, if you are discreet and don't stink up the neighbourhood, I doubt they would go looking for it. They would only act on a complaint (from cranky people next door looking to get at you).
I think urban authorities would be much more aggressive due to the concentration of people. Other active enforcement locations would be adjacent to lakes/rivers, very high water tables, very shallow soil or high flooding hazards.
If you have a permanent dwelling, you could be ordered to install an approved septic system -- a spendy prospect.
Well, that depends on where you are and how motivated people are to get you. Vague enough answer for you? The norm is fines. Fines fines fines. Then they'll force you up to code. And if you don't pay the fines you could face the possibility of jail.
I do have a fully compliant sewage system complete with drainage field. But, I find it fascinating that a few years ago our water department sent out a survey form asking what kind of sewage system was in place. It is amazing the number of people who reported themselves.
Good decisions come from experience. Experience comes from making bad decisions. Mark Twain
I imagine it depends on how many bylaws you're breaking and how much they want to teach you a lesson. You could be breaking building bylaws (plumbing, house not to code, etc) and/or health & environment bylaws (illegal toxic waste disposal, etc).
I know it's a pain, but I would advocate for learning what is required to conform in your jurisdiction, and if it's not possible to continue legally, perhaps challenge unreasonable bylaws that prevent you from taking the high road. I've started the process in my city and it's taking a very long time for even my initial questions to get answered (but I haven't gone the call/inperson route yet).