Mysterious mildew smell

Came back from a 3-week trip only to find my apartment smelling like mildew for some reason. Had the windows open for 24 hours, and it still smells like that. There are no stains or discolouration anywhere. Before my trip, I did some dry hoovering, closed all the windows and left the boiler and thermostat on 21°C (I live in Scotland, and the weather was still cold enough that I was apprehensive of turning the heating off while I'm away to avoid any mold growth and such). And yet, the mildew smell persists. What can I do to find the source and get rid of it? Thanks in advance for your advice.