When the septic is serviced and all solids removed the effluent will also be removed. It is normal for the tank to refill with water within 4-5 days. It will fill up to what we refer as “normal operating level”; the septic system does not begin to function until this happens. Once the tank is up to the level where the effluent flows out thru the outlet tee, the system begins to function once again. However, if the operating level climbs higher than the out flow tee there may be a problem. Septic tanks are just a vault to collect solids and allow for some digestion, when you flush a toilet with solids and water the tank is designed to trap the solids and send the effluent out to the leaching area. Over time the solids will reduce in volume and settle to the bottom of the tank. This is called sludge and it is heavier than water. There is a scum layer that floats to the surface, this is lighter than water and in time can reduce in volume with proper bacterial action. This is why you pump your tank regularly, to remove the solids. If neglected the solids can build up in the tank and flow to the leaching area which will shorten life expectancy. So the water in the tank is not an issue unless your leaching area is saturated and/or over loaded.
So the tank refilling with water is a normal and expected occurrence.