You do not have to be actively watching when they break the door down. The offence, IIRC, is "receiving", which is why even recording counts as needing a licence. If it goes to a prosecution, as usual, it'll be proving you received "beyond reasonable doubt" that is required, not catching you in the act. So whether you get convicted or not will depend on the overall case made, of which no doubt finding a set connected, with power, and tuned, will play a big part. If that set is turned on, with both power and aerial connected, and is tuned, it's receiving.
So your case would have to be that "reasonable doubt" was not reached, that the set had not been receiving, which means not turned on, whether you were caught actively watching it or not. No two cases are ever identical, and magistrates are individuals reaching opinions, so there is no 'one size fits all' answer to that, but I can tell you how that'd be likely to go with one magistrate and that set being there, powered and tuned, would be "persuasive" evidence against you.
But it isn't powered on (unless you leave your electronics switched on when you are out?) and even if it was, it would be on HDMI1 watching Netflix/Amazon Prime/Streamed content of some sort. Yes it is plugged in to an aerial and yes it is tuned in but it is not being used, so surely some sort of proof has to exist that it is being used? You cannot just connect the dots and come up with the answer of "guilty". Never the less, was just a hypothesis.
Devils advocate
Does a 20 year old TV sat up in the loft requires the licence fee if it's powered off
What about if its powered on?
Absolutely not.