Oh, crap, I think I discovered a bug with the distance expression - mostly to do with picking.
Simply put, I tried a little test with placeable turrets, all of which have a range, which is calculated using the distance expression... Unfortunately, when using the "compare two values" condition, I couldn't get the turrets to fire, period. I messed about with the condition, and discovered that it only worked with the "greater or equal" sign, but then I also noticed that, when I tried to debug the output value for the distance expression, the first one to print was a number that was way too big to be possible. And then I realized that I kept the original instance of the turret object outside the layout - I brought it in, and did more tests, and I realised what the problem was.
Simply put, the "distance" expression plus the "compare two values" condition will ONLY work if the original instances of the two objects are involved - once the TestEnemy gets outside the TestTurret's range, all turrets will cease firing, even if it's in range of the other turrets, or there are other instances of TestEnemy in the original TestTurret's range.
Example cap (click on the black stuff to spawn turrets)