This is a question I have often asked myself on Christmas Eve...
I usually leave them in the boot of the car camouflaged by old blankets though also in closets (inside coats) and places like the cupboard over the fridge.
My MO for Santa Clausing when the DCs were young was to pack them off to bed and get started when I figured they were sleeping. This meant I was starting very late as they were determined to see Santa and really good at avoiding sleep. Now that they have all pretty much grown up they hit the bed with much less fuss. I still only get down to business about midnight or later however.
I normally wrack my brains trying to remember where all the stuff is even now, as I end to buy throughout the year. I once found a lovely Snow White porcelain doll under the basement couch in July.
When they were little I made sure never to leave anything upstairs of course, and also not to make too much noise with plastic bags. Nowadays nobody minds.
I never wrap anything, never have, but it still takes time, and one of the most time consuming parts of the job I find is picking off sticky price tags from things. Each person's stash goes under the tree in its own little pile. I have little 4 inch stockings, each with a DC's name embroidered onto it, that hang on the tree until Christmas Eve, and these I fill with chocolate coins and Santa figures from Aldi (yummy German choc). Before we had a fireplace I used to place each little stocking with the pile for whoever owned the stocking. Now we have a mantel and I do my best to stick the filled stockings to it at about 1-2 am.
In the past I would often be woken by scuffling sounds and 'ooooooh' and 'aaaaah' and loud 'sssshhhhhh' noises, it seemed to me about an hour after getting to bed. I once woke up at 8 to find three little DCs fast asleep on the cold, hard wood floor under the tree. These days, we all wake between 8 and 9.