I would say...not a big deal...I would look into this thing however, like you say, you don't know long it's been like this and it hasn't caused any issues and, frankly (and don't take this to heart young people) it is an extremley bullcrap issue anyway...if it fell of into you timing chain assembly you wouldn't know about it? A dual roller-chain timing assembly held together with a plastic cover? No...absolutely not....it's a cover that probably got tossed by a mechanic at some prior servicing.....he decided to toss it rather than charge (and piss off) you for it to be replaced ...forget about this....