I've just had this on my A4. The non return valve (apparently a spring and ball bearing)was leaking very slowly and released pressure on the timing tensioner. Local garage drained oil to do another job involving oil cooler, left it standing over weekend and when they started it up after completion, the timing jumped 3 notches and the subsequent repair (for which they deny all liability) was in excess of £2k.
This can apparently happen if the car is not started for a while, allowing oil to leak and de-pressurize the valve. Of course, there's no way of knowing if you have a worn valve, so imagine parking at an airport for a couple of weeks, turning your engine over and immediately needing a rebuild.
Hope this makes sense...I'm not a mechanic.