Border crossing

For a temporal visa holder, driving a US plated vehicle with a current TIP, is it required to stop at MX immigrations on the way out and back into MX again? Only going across border for a few days. I assume it might be the law but would they even know if you didn't stop?

Yes, you are supposed to stop and it's a good idea. If your vehicle should unluckily get wrecked while in the US, you wouldn't be able to bring another vehicle in with a new TIP if you don't check out, because as far as Mexican customs can see, you never took the original vehicle out of Mexico.