Why does Obi go to visit the mines?
I’m sure the episode explains this- he’s suspicious that they may be active and Death Watch, and goes to check them out.
Why does the the mines level of operation indicate Death Watch activity?
They’re clearly being used to produce new Mandalorian armour- the same armour that the commando who attacked the Republic cruiser was wearing. He trusts that Satine would be telling him if the peaceful Mandalorians were producing their own armour again.
It’s that last bit though, that they’re now being used as factories that I don’t recall being highlighted in the episode. It’s an effective shot when he sees that they are producing the armour, but his argument for leaving Satine in the first place was to see if the mines were active or not. There’s nothing previously to suggest that the Concordians mining is suspicious activity. Mines aren’t foundries or factories (at least not automatically) so it’s a leap to go from ‘there’s mines on this planet’ to 'I bet they’re making supercommando armour there).
As I write this I know exactly what would have cleared it up… The word ‘Beskar’. Beskar mines would indicate armour production. It’s a shame the word is never mentioned in TCW.
I’ll look at episode 1 this evening.