DJM, had same problem with DS & someone suggested straps might be at wrong height. Adjusted them to a notch higher & this did the trick (the top strap should be just above or on shoulder level). Hopefully this will work for you too
I've had this problem for the past week. I borrowed a Maxi Cosi Tobi off a friend and my 2 year old can easily get his hands out of the harness. Which he did twice when I was driving at speed along the motorway - truly terrifying.
I heard that manufacturers recommend disciplining your child as a way of getting them not to do it. and I guess after him doing it once more, and me going ballistic at him he has not done it again. But (I work in Mothercare) when customers come in with this problem I recommend them getting a normal walking/highchair harness, and doing it up back to front over the top of the 5-point harness. Its quite fiddly for a small child to be able to undo it, and it keeps the straps closer together, so its harder for them to get their arms out of. Plus, if something awful happens, then the accident response team can still easily slip a knife through the harness to get the child out quickly.
Its a scarey problem, and whilst not common, I'd say I've had about 5 people in the last year who have had this happen to them. (+ me too!) HTH good luck.