With the ratcheting tensioner on these engines, it would seem to be close to impossible for the chain to "jump". Of course, nothing is impossible, but I think this would be about as close as you can get.
mplefleur, you're wise for removing the lower cover to learn more. Be patient and hang in there.
BTW, I expect that the balancer on this car is indexed by two round pins, not a woodruff key. If it is, you will need to put it back in place without the pins, then put the pins in place. Too many people just drive these things on with the pins in place by turning the crankshaft bolt. Many engines/crankshafts have been ruined by this method. Proceed carefully. I have not removed the balancer all the way from an M103, but the diesels are like this, so I just wanted to warn you about this while I'm thinking about it.