If it's access to public drains or sewers then it's not unusual for them to be on private land but they are usually in an accessible place. It's possible that the garage was constructed over the top of this after the drains were laid, possibly the house did not originally have a garage. Years ago I worked with a sewer surveyor and we had to lift a cover in someone's garden but it had been turfed over! We found it by accurate measuring and sure enough it was there. We had to cut the lawn back and lift it. We put all the grass back though, the owner seemed surprised it was there but they were happy our restoration of the lawn!