The only way you would ever find out is to take it to the vets for tests on poison or whatever, bit of an expense with no reward bar your own curiosity and wondering what might’ve harmed it.
Sad. I can empathise.
Where I live it’s the roads that have done them in and I think away for good. Spotted one in April and it was sensible, it was on the road quiet at the time, but stuck to the curb like glue. Then last week saw one killed close to where it had been living, near a park. Run over. Prob same one, not many left round here.
Hopefully you can get more back into your garden though.
