Hooking an ammeter to it to find your draw is what you need to do, once you have the meter hooked up go press on the glove box door lid and see if the draw goes away. Most of the mysterious draws I find on the 850's are from the glove box light staying on. By the way, you are looking for a number less than about 30 milliamps, that is .030 amps. If you are at 30 milliamps or less then you are good to go. If it is more, like 200-750 milliamps then you need to start looking for the consumer that is still on, i.e. a light bulb somewhere.

This shop that told you that you were not driving the car often enough and that was the cause of your problem? Please tell me that you did not pay for that diagnosis and if you did lets hope to god it was after they checked it out for an excessive current draw and found none. Otherwise if you paid for a diagnosis like that without a current draw test then I would never go back there. On the other hand if it was free advice then you got exactly what you paid for. Don't get me wrong, an 850 left to sit for too long will drain the battery as they do have a high parasitic load value but I only experience that in extreme cases like really old people or folks with jobs like flight attendants that leave them parked for extended periods. 30 milliamps is actually a fair amount of current to drain from the battey over a long time. If you look at any of your red engine cars they are down in the teens and lower.

So check your current draw, if it is over 30 (like into the hundreds) check your glove box first and if that is not the case then pull the fuses one at a time and replace them until you pull the one fuse that makes the draw go away. No you will have it narrowed down to one circuit and you can diagnose it from there.

Mark