Medakulw

So I stopped working for my employer in September 2023 in order to go travelling in South America. I was employed for an NHS health board through a locum agency. Since leaving I have been receiving weekly payments which appear in my HMRC app account as £0.00 from the employer but -£XX.XX (the amount has varied) I have been receiving these weekly since I left the UK and didn't notice until about a month or so ago. I returned to the UK last week and am not yet employed. In the app it says I'm still employed by this employer so maybe they did not inform HMRC that I left. I'm wondering what these payments are and if I'm going to have to pay them back