Apologies if I'm being really stupid but I'm not a very close viewer of the programme but I'm sure I've seen the silver medallion the captain used to lock the door in another episode?

Can't remember seeing it myself but I may have missed it!

think we're in danger of seeing far too many patterns and re-occurrences that just aren't there.

Are you thinking of the fob watch used by DT and by the Master, to contain their memories of being a timelord? That was my first thought on seeing it, but it seemed to have no significance beyond being a reflective surface so I think any similarity is coincidental.