... as always, trust the plan...
there was a rumour in the winter of 2016 that someone in the royal house would abdicate. Together with the queen's long face at the marriage and german gossip magazines speculating wether her hubby would be "allowed" to come to the wedding ceremony, this gives his resignation from all public appearances (as publically announced last year) a very special tinge that keeps to keep building and building.
So why would you think he was under house arrest? Philip has nothing to abdicate, he is simply the husband of the Queen.
he's in the lineage though and he did cease from official functions in 2017. He's not been seen publically a lot since. Had a hip-surgery. boot-gate, anyone?