So, this is funny. After placing one of our calendar spreads today, I noticed that one of the underlying PUT contracts was not being updated in IB TWS, and it was still showing yesterday's close price (prefixed with a 'c'). Interestingly, Time & Sales showed trades and Yahoo Finance also showed a different LAST price for that contract.
After talking to IB Tech Support, it turns out that they don't update the LAST price of the contract if the trade belonged to a spread. That's what the yellow arrow means in the Time and Sales panel.
Did you know that? This essentially means that I don't have a real-time P&L view into my spread... I'm flying blind...