I believe that the septic tank covers are really heavy concrete ones. (We looked into it a lot when they were searching Spedding's property. If you see those search pics, you will see a local septic cover.)
I think it would have been evident if one had been lifted/moved. And may be impossible for a young tike to move one of the covers. Regardless, they did stress that the septic tanks had been searched in the Benaroon Drive area.
It also comes down to what type of septic system they are using. If an old fashioned septic - access hole is likely to be buried under the ground, and should have a concrete lid covering it. One we had in a previous house, lid was curved (as top of tank was curved/rounded so impossible to flip over). There may/may not be a sand filter tank that might have an exposed concrete lid on top - the previous house I lived in had a concrete pipe coming out of the ground with the concrete lid sitting on top (for the sand filter).
If it's a waste water treatment system, one or two tanks might be partly above ground, with the lid exposed. Some lids are concrete, some are plastic.
Our treatment system has 2 exposed concrete lids to 2 of the tanks. (3rd tank is buried underground and that is where the sludge settles that needs to be pumped out every few years).
The concrete lids are extremely heavy to move. The tank that holds the treated water (which is pumped to the garden) has a lid that is 2 semi circles, and heavy concrete. Very hard to move (but can fall in easily and are a bugger to retrieve! Especially if you don't have the right tool that hooks into the small metal handle) It's easy to tell the lid has been moved recently - fresh garden dirt/bark/grass will stick to it, and the concrete rubs off of a little as it slides off.
To check this treated water tank - all one needs to do is to push the pump down to the bottom, and the water gets emptied to the garden. If the water level rises rather quickly (ie if a body fell in) it would likely trigger an alarm and there would be a record of that date/time that water level was too high. This 'history' is checked by those that come service the septic.
2nd exposed lid is where the waste comes in and gets treated. Lots of things sitting inside that tank... like hoses and blocks that hold the good bacteria that eats the waste. There is a circulation pump in this one - and it sets off an alarm if it stops working.
3rd is completely underground and needs to be dug up every now and then, and have someone come in and de-sludge. (though we've had ours uncovered at times due to ongoing faults with system - but this lid sits snugly on top, and is curved (like a plain septic tank) - it is impossible to flip over (otherwise it would fill up with the dirt.
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Every 3 months our system is serviced by a professional - they remove the 2 exposed lids and take water samples of the treated water and check the box for any alarm history.
