That's the purpose of the magnetic drain plug, to collect the those minor pieces from manufacturing during the intial break in period. It's not unusual to see these plugs fully incased, it would be unusual if you were to drain anymore then that. Now that you've changed the fluid, you should be set for life. These grease boxes are very heavy duty and you just don't hear much about them.
If you have deeper concerns, call FreightLiner Customer Service 800-FTL-HELP.
07 Monaco Dynasty 43Q4
"An Unbelievable Coach"