Science is the belief in the ignorance of experts.
When an enemy is in the staging area, does he get a shadow card if he is not engaged with anyone (ie, the threat level is too low).
No. Only engaged enemies get shadow cards.
If so, does he or other enemies that are in persistent combat (ie: from the previous round with damage on them) get additional shadow cards?
No. Shadow cards don't accumulate over turns. The rule book explains:
Shadow Cards Leaving Play
Shadow cards remain on the enemy to which they were dealt throughout the combat phase. If that enemy leaves play, discard its shadow card from play. At the end of the combat phase, discard all shadow cards that were dealt this round.
You should also look on page 18 which explains:
At the beginning of the combat phase, the players deal 1 shadow card to each engaged enemy. Deal the top card of the encounter deck, face down, to each engaged enemy. When dealing cards to a single player’s enemies, always deal to the enemy with the highest engagement cost first. Cards should first be dealt to the enemies attacking the first player, and then proceed around the board until all enemies have 1 card.
The last sentence should have said "... until all engaged enemies have 1 card".