Help need your opinion!?!?!?!?

so me and my girl have been together for a while, im 22 and shes 26, shes got 2 kids, and we have been together for 6 months now. she lives 4 hours away from me so its a bit of a long distance relationship, so far she has showed me no reason not to trust her or anything. but we dropped her car off at the auto repair shop, and i noticed on her keys was a key ive never seen her use. i asked her about it, and it turns out its her ex-boyfriends(the one she was with before me) key to his appartment, when they broke up, he did it over the phone so they never gave eachother's belongings back and have as far as i know no communication at all.

she says she completely forgot it was on there. and at first it didnt bother me at all, and i kinda laughed about it, but now after a couple hours of it stiring in my brain its really bothering me. they where together on and off for 3 years and i was told by all her friends and family that she really wasnt happy at all, even she has said those past couple years has been pretty bad.

she seems very happy when shes around me and her friends all are glad that im with her because she hasnt been this happy in years. but even with this knowledge it still seems to bother me. do i need to be worried at all that she still has his appartment key on her keys?