Do the turrets have the most random detection you've ever seen. 90% of the time ill place one and an enemy can run straight past it without the turret even shooting once at them, then once every now and then it will get some ownage kill. More times than not though it wont even detect and enemy 5m away in plain sight...

I agree, I have had some I can run right past to get behind them and destroy them and others have shot at me from almost half the map away.
They must be fitted with human brains instead of electronics and occasionally you will get a Derp turret.

I've found the same thing, mine just don't seem to shoot anyone, however I've found that although you can run past one without it taking much notice of you, if you shoot it the turret will return fire.

If you shoot the enemy in the turret's sight, drawing his attention, the turret will usually track him and waste him.