They are attracted by the odour which, transmitted through a thin crevice, hardly reaches my nostrils.
They explore the metal receptacle for some time, seeking an entrance.
Finding naught that enables them to reach the coveted morsel, they decide to lay their eggs on the tin, just beside the aperture.
Sometimes, when the width of the passage allows of it, they insert the ovipositor into the tin and lay the eggs inside, on the very edge of the slit.

Whether outside or in, the eggs are dabbed down in a fairly regular and absolutely white layer.
We have seen the Bluebottle refusing to lay her eggs on the paper bag, notwithstanding the carrion fumes of the Linnet enclosed; yet now, without hesitation, she lays them on a sheet of metal.

Can the nature of the floor make any difference to her?
I replace the tin lid by a paper cover stretched and pasted over the orifice.

With the point of my knife I make a narrow slit in this new lid.

That is quite enough: the parent accepts the paper.
What determined her, therefore, is not simply the smell, which can easily be perceived even through the uncut paper, but, above all, the crevice, which will provide an entrance for the vermin, hatched outside, near the narrow passage.