I see the same behavior and haven't found a good reason for it so far. Some personal theories that didn't work out:
- Indicates when the ferrets pick up a large pile: No, they can glow for small change and then silently pick up a large pile.
- Every X amount of gold picked up: I kept a close watch on my gold at hand and the ferrets' glow but this also shows no correlation
- Some kind of interaction bug with Enchantress' Powered Armor skill: Had this conjecture since I first realized it while with Eirena, but have also seen it happen in multiplayer
One conjecture I have (that is untestable) is that it is simply a means to occasionally identify your ferrets, which makes sense in a team game with multiple DH/ferret - however, DH pets are non-targetable and pretty much fire & forget, so that doesn't make sense either.
Things I need to try:
- Checking if it boosts GF as @Xenogard posted - doubtful of this; but it may just be a periodically refreshing 10% buff? (afaik ferrets add 10% GF)
I did manage to replicate what @DedBob posted, that ferrets glow repeatedly when they can't get to a pile of gold nearby. Not sure if that is the only reason as they also glow when picking up piles of gold right in the middle of the floor - this could just be a pathing hiccup, but may indicate other reasons to glow as well.
An observation from the inaccessibility of piles is that ferrets may not benefit from +gold radius items on the player, and have 0 radius always.