You have to also consider the P&L of your trading day. Whether you won or lost, it could have additional funds or fewer funds for margin. As my colleague mentioned above, the margin itself on ES doe snot change day to day, but the number of contracts you trade may.
I found out that somehow an order made it into my broker from SierraChart but Sierra did not show the order. I contacted the broker and fixed it. It was never filled, but still open. The dashboard was correct, SierraChart was not. Scary.
Ok I found the real problem. Operator error. I tried to buy market order and interface was set to limit with a price of 0.00, so it sent a limit order for 0.00. When I clicked on Buy it did not give me an error or indication anything went through, so I thought it just wouldn't let me do that. I didn't notice until until I brought up the broker's dashboard. Really confusing.
Discretion of the broker matters: https://ibkr.info/node/66 "Finally, note that SMA is a Reg. T concept used to evaluate whether securities accounts carried by IB LLC are in compliance with overnight initial margin requirements and it is not used to determine compliance with maintenance margin requirements on either an intraday or overnight basis. It is also not used to determine whether commodities accounts are margin compliant. Similarly, accounts which report negative SMA at the time each day when overnight, or Reg.T initial margin requirements go into effect (15:50 ET) are subject to position liquidations to ensure margin compliance." But long, long ago, I remember learning that it might feel that way.....