There is a damp line being formed behind the toilet commode, as seen in the picture. If we stop using the toilet for a few days, the wall dries out. 
Any suggestions on where the problem lies would be appreciated. 

[![enter image description here][1]][1]

Edit: 

I've added pictures of what it looks like with the lid off (the lid is usually left on) . I don't see particularly see any signs of condensation on the inside. Also attached is a picture of the wall right behind the commode so you can see how far down the wall the damp line runs (annotated the level with a red arrow) 

The toilet is on the 1st floor of a 2 story home. However, on the 2nd floor, there are no drain pipes right above this toilet that I can think of (there is a game room right above). 


[![enter image description here][2]][2]
[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/cmc3J.jpg
  [2]: https://i.sstatic.net/T2s31.jpg
  [3]: https://i.sstatic.net/vLW1m.jpg