Also, I have found the "change address" online for the missing BTC. It was sent from one of my bitcoin addresses in 2014 to two separate addresses.
and it seems bitcoins were sent from that address (plus 3 others)
That sounds like a fairly "normal" transaction... you've used a four previous unspent transaction outputs (UTXOs) as "Inputs"... They have been combined into a total amount and have then been split into two "Outputs"... One output, being the person/address you're sending coins to... and the other output being a "change address" that is created automatically within your wallet to receive
the leftover "change".
One of the two addresses is listed in my wallet and when I run dumpprivkey it returns back the private key.
That would most likely be the "change address" in your wallet...
The other address however that was sent (at the same day / exact time) returns back nothing when I run dumpprivkey. Is this b/c my wallet is out of sync?
No. That sounds like an external address that you were sending coins to.
It's unlikely that both addresses would be in your wallet, unless you sent coins to yourself.