An uncommon source of tank leakage can be due to incorrect positioning of the fill valve hose.
If the end of the hose sits below the waterline in the toilet tank, water can siphon into the hose, through the fill valve, and into the bowl.
Siphonage through the fill valve hose would be slow, perhaps one drop of water every 3 seconds.
The slow leak is not quickly noticed, but results in the fill valve turning on as frequently as every 15 to 30 minutes.
For tanks with non-canister flush valves, the refill hose may sit too low in the tube if the clip is not holding it in place.
To correct, install the clip (part 51261) to secure the hose in position on the tube.
Attach one end of the clip to the end of the hose and the other to the edge of the tube.
If a clip is not readily available, a paperclip can temporarily be used to hold the hose in place.
NOTE: The paperclip solution is only temporary. The paperclip can corrode quickly, and must be replaced with a stainless steel or copper clip as soon as possible.
For tanks with canisters, the refill hose may be disconnected from the flush valve port, resulting in the end of the hose sitting below the waterline.
To correct, reconnect the end of the hose into the port at the top of the flush valve.
More common causes of toilet tank water level issues are:
Leaky flapper
Fill valve adjustment too high
Flush valve chain too tight
Dirty seal or defective valve, preventing the fill valve from turning off completely
Leak between the tank and valve base
Flush valve seat is nicked
Line pressure spike, causing the valve to turn on Cracked tank