DGA 2500 smoke unit not on zero. 5 years old unit .

I had a BMW diesel in for its first mot and limit was 0.09 (lowest I have seen so a while)and it had done 26000 miles. Entered it all in and tested it and it failed all the  runs at either 0.20 or 0.21. Tried it again with same result as it just seemed odd the reading it was showing . The other tester came in the bay as I said it seems strange the readings I am getting. Entered it all in again but this time after the tester showed it had zeroed we noticed as you picked up the head unit was then showing 0.16 to 0.18 before you even got near the car. Putting the head unit back into the cradle it again showed 0.00 or 0.01 but again showed 0.16 or more every time it was lifted . Tried every thing Snap-On said on the phone from removing unit battery to resetting every thing. With no real luck and they said they would send some one out .  Suddenly it seemed ok and seem to show 0.01 as it went in to the exhaust and the car passed 0.04.
   Snap on chap calls in  a few hours later and said I know what it is and I have seen it 2 times in the last few weeks. Turns out 2 grub screws holding the tube tight in the smoke head unit are slightly loose which means the laser moves changing the zero when it is moved . Every time I tested it yesterday it now stay on 0.00 in the cradle ,walking round with it and putting on the workshop floor. Having been told before in the passed on the phone to Snap on what ever the unit shows is 100% right I know now it is not correct. Still 4 months before the Diesel part needs its yearly calibration is due . Not sure how long it has been like this as most tests seem around 0.50 or more I see and not many get  close to failing. Still reading people are seeing vehicles as low as 0.01 I know know to watch the screen for what the unit it showing before it is in the exhaust .