Older Kitten pooing in garage

[ilovebagpuss]

I wonder if anyone can help with this issue please. Our female kitten is now 10 months and happily goes outside. We kept the indoor litter tray just because there was occasionally a night where she would use it.
She does not seem to understand where to go In the garden and keeps going in the garage where she can find a dark corner.
I've tried putting some pooey litter on a patch in the garden we can clean after her and there is loads of space for her to go.
Bizarrely she doesn't come in to poo In the tray which she could do!
Any ideas? Our older cat never had this problem could it be something to do with his territory? Thanks

Maybe just put a litter tray in the spot she likes. She's chosen it for a reason.

I did think about that but I'm trying to wean her off the litter tray. It's also where my DH keeps all the tools for his business so not actually idea for the tray.
I've never had an outdoor cat that won't go outdoors so it's a bit odd.