When a septic tank needs to be cleaned out, it's usually noticeable by the waste backing up in the toliets, sinks and/or shower/tubs.
Maybe one of the tenants complained they were having issues like the above.
I believe it's highly possible someone may have checked the septic to see how full it was, forgot to replace the lid and sod, or walked away for a few moments/or few hours and remembered, went back put the lid on bolted it down and replaced the sod not knowing Noah had fallen in.
As for the tip being called in, I don't believe it was for the reward money. It could have been because someone got so busy with other things in life, chores/work it didn't dawn on them until a few days later they'd removed the lid to the septic and replaced it. It could be they were so distraught over what may have happened, this was the reason for calling the tip line. Just a freak accident. All JMO.
It's not my place to judge the parents, so I'll just sit on the fence.
RIP Noah...