Plumbing – Why is the hot water tank randomly emptying through the overflow pipe

If there is not any backup evident at the outdoor clean-outs or tub drain when sinks and toilets are flushed together, then I would say the septic is not slow draining and air bubbling into the toilet requires that the soil vent serving the toilet be plugged some other way. Since the toilet vent line was recently cleaned and found to be plugged, I would surmise that the vent has somehow become plugged again.

Offhand I can think of two possibilities:

(1) The toilet soil pipe has a sag near the connection to the soil vent such that the branch to the vent tends to collect sewage and become plugged;

(2) The toilet soil vent is connected to the drain line incorrectly (incorrect wye, installed backward, etc);

There are companies for hire that can run a camera through the lines to find the problem if you are so inclined.

A- This is not an "overflow" pipe.

B- It is not connected to an "overflow" valve.

C- It is a drain pipe connected to a Temperature & Pressure Relief Valve which is not only a legal requirement, but a critical safety device to prevent an over-pressure event (boiler explosion- google it)

For years and years, it was common to see no pipe at all. The pipe is a great idea whether or not it is plumbed to a drain or alternative location because if the valve activates it is less of a hazard to humans when scalding steam and water are directed downward and away from hapless victims. Some (if not all) localities require that they be plumbed to the exterior of the building or to a floor drain if the appliance is located in the home. Appliances outside of the home (as yours appears to be) may not be subject to the same requirement. Check with your local building code authority office.

P.S.- there should not be water coming from that drain pipe. If there is you may have a defective TP Valve (or worse, a malfunctioning boiler). Call a plumber.