I'm in Arizona (not Tucson area, so take anything I say with a big dose of salt as I don't know Tucson codes) but I've seen a few covers of that general kind on old septic tanks, and similar on newer. What cries out "Septic tank!" to me is two access hatches seen in the drone video. This site shows a few photos that explain the layout better than I can.
Septic tank collects sewage and treats it by bacterial decomposition and is generally located underground. For a better septic system, a good septic tank is important which depends on the type of m…
theconstructor.org
They vary a lot, I've even seen square access hatches. ones. I've seen small covers on newer ones, BUT, the ones I saw were small hatches within manhole sized ones. I don't recall ever seeing a small cover (smaller than human sized) as the only way in. And BTW, I rarely see locks on them, or even provisions for locks, though I personally think they should be locked.