Have a 2013 GLK350. At night in the garage, lock the vehicle with the key and the next morning the car is unlocked with the seat position moved. It is just the wife and I at home, neither of us sleep walk so know we are not doing it! LOL

Are only thought is to have them flash the Keyless Go software.

 

Assuming you both have separate keys.
1. Try removing the battery or sticking one key in a shielded container overnight. Next night do the other key. Am guessing that if the seat position is changed, after you lock the car with your key, its the wife's key casing the seat switch.
2. If that doesn't work, do you have neighbors with German cars? Mercedes uses a Siemens system for its keys and so do other German cars.
It's possible that a neighbor has a key fob that's causing the issue. Not very likely, but possible. If that's the case, you might have to have your keys re-coded.
3. Last idea is you have someone with one of the black boxes that's trying to steal your car. There is a rash of this in Europe and all cars using Keyless Go and other similar systems are vulnerable. If this is the case, try putting both your keys in a shielded container when they are not in use. Amazon has RF shielded pouches for this for less than $10. A thief can stand up to 30 feet away, outside your house, and unlock your car and drive it away with one of these devices. Mercedes is supposedly offering keys for newer vehicles that let you disable keyless go with a button on the key. I haven't been able to get any straight answers from my dealership on whether the newer keys will work with older GLKs.
4. If its none of those things, you might then have the dealer do some tests on the security system.