Baxi Combi 133 - how to stop the heating coming on randomly?

Hi
In the last couple of days we have noticed our lounge radiators getting warm at random times of the day. Our Baxi Combi 133-100-80 HE Plus Condensing Combination Boiler is currently set to summer and there is no timer set so the heating shouldn't be coming on at all. However several times we have woken up to or come home from work to lounge radiators fully on and have had to turn off all the other radiators in the house to avoid walking into a heatwave! Any ideas why this is happening? The thermostat is turned fully off as we don't need the heating on yet. We aren't able to turn the lounge radiators off as they don't have control valves. We've not had this problem before.