Flies, should I be worried?

[Borderstotheleftofme]

I have counted at least 20 what look like young bluebottle/house type flies in my garden congregating on my plant pots and windowsill.
I’m a bit concerned because there shouldn’t be anything to attract them, i have a very clean, tidy garden. Mostly gravel. No rubbish, small dead animals like little birds or mice, no animal excrement.

What’s attracting them?!
Should I be worried?

I know you obviously do get flies but it’s a large amount for seemingly no reason, out of nowhere (they weren’t here yesterday) on a pretty cold, damp day too.
So can’t even blame hot summer weather.

It seems odd to me to be worried about flies outside in the garden where they should be. Just ignore them and I am sure they will go away soon.

It just seems odd to me for such a large group in one very small area?

Check under the patio

Possibly some left out food or something dead under a shed or in a gutter? Also cat or dog shit?