what happens if Fedex tries to deliver when I am absent? should I somehow warn them?

This sounds like SmartPost to me, which is a completely different service from FedEx Express (or Ground) and afaik doesn’t offer a signature required option. Did you specifically select FedEx Express with signature required when you ordered? Most merchants don’t let you be that specific (especially for low value items like a CD); they’ll let you choose the speed and then they pick the carrier and service type to get it to you within that time frame.