I think it's more a privacy issue, like CoinBase could start blocking PD withdrawals etc. Although I assume the "hot wallet" is actually just a collection of addresses that change all the time.
There is also no reason for a public cold wallet since there are no public investors risking money at PD (unlike those other sites that you mentioned.)
Pretty sure you can just get the IP of the node running the Bitcoind server to get a full list of addresses associated with PD. Or something along those lines... Know there's a way though
Yeh, definitely. What I mean is this: betcoin has (or used to have) 1 hot wallet bitcoin address. Most withdrawals went through that address. Then some coinbase accounts were frozen because of withdrawals from there. Basically I mean: they won't make it too obvious to detect, even though there are ways. Eg: it was kinda suggested that PD could have 1 public hot wallet address so people can see if it's empty or not, which would be bad.
The idea about monitoring the IP is this: connect to as many nodes as possible. You could probably connect to a lot of them, like blockchain.info does, but probably not all. Then you can probably detect the PD node after a while and you can assume most of the transaction they first relay before all the other nodes, are PD withdrawals. This isn't 100% correct though, since it will relay transactions from other nodes too and for sure you are not following all the nodes. Basically I am pretty sure coinbase doesn't and won't use this method for detecting transactions they don't "want".