How to detect the absence of an RFID tag?
I don't think it is particularly practical, unless you make your own RFID scanner. The off-the-shelf ones generally only respond once when a particular card is in range, so you can't even detect if the card is left next the reader, or not.

Also, as the range is usually quite limited (a few cm) I don't see how it could tell the difference between you being out of the house, or taking a bath.

What might work better would be to have an active transmitter (eg. a nrf24l01) in your pocket, which every minute or so sent a "I'm home!" message to a central place. The absence of such a message after 10 minutes could indicate you are out.

The other approach would be to "check out" with your RFID scanner. I did an RFID event management system a little while back. You use a special "admin" card to indicate that a person was leaving an event for a while (a pass-out). You could do that, or have a button by the door, which you press. The "I am leaving" button.