As the car has been idle and there has been a lot of rain lately, it's probably got in via the scuttle below the windscreen. Some scuttle drains can block with leaf ru66ish (also a lot of that now). Otherwise perhaps via the passenger door-card (not the edge seals) ?

Edited by Andrew-T on 04/11/2024 at 12:51

Since its only the passenger footwear, drivability wont be compromised.

Perhaps not so. There's going to be electrical connections under the seat for sensing occupancy to activate air bags, belt pre-tensionsers and other such gubbins. There may well be ECU type electronic modules down there as well, as it seems to be a favoured place by some manufacturers.

The problem with leaks is that the entry point can be some distance from where the water collects. I would check the obvious things such as leaves blocking drainage channels under the bonnet. Check the sunroof channels, if fitted. Windscreen seal. I had an old Golf once and that would leak through the door card because the polythene membrane would crack after a good few years.