As I saw it absolutely he'd give a big old radar return - at this point probably larger than an aircraft his rough size due to his much rougher surface, the way he's built would really stand out on a search radar.
However - this is happening in the northwest of Scotland, in the middle of some of the roughest terrain anywhere on Earth - the peaks aren't nearly as high, but to find as much variance in altitude within the same horizontal distance you have to go to places such as the Himalayas.
I picked the location for Hogwarts for this fic - which I've since consistently used for all my subsequent fics - with a great deal of care. It had to be further down the railway between Mallaig and Fort William due to the prominent inclusion in the films of the incredibly visually distinctive Glenfinnan Viaduct: that curved bridge with the multiple arches. To a Scots railfan, it ties Hogwarts to the northwestern half of the Mallaig line as resoundingly as putting the Statue of Liberty in your establishing shots ties something to New York.
I settled on Loch Morar largely as there is an area of flat land at the northeast 'corner' of the loch sufficient to provide a location for Hogwarts itself. This is at an elevation of roughly 40 feet above sea level - I understand that the surface of the loch is about thirty feet above sea level. North and east of this area, the land rises up very steeply - about mile north of my chosen location for the castle there is a ridge-line standing at roughly 2,000 feet altitude with higher peaks such as Sgurr na ba Ruadh, and similar ridgelines surround the north end of the loch to the south and east, in places rising to peaks over 3,000 feet; meanwhile to the west there is only a very narrow corridor giving line of sight out to sea, and that's got a two-and-a-half-thousand-foot-tall island smack in front of it over just a few miles of water.
Here's a topographic map of the loch - you can click the map at any location to get an altitude, and zoom it and pan it around.
http://en-gb.topographic-map.com/places/Loch-Morar-785465/
There are some topographic differences between reality and the fic, most specifically the crack in the hills on the south side of the loch where first the goblins and later the Grangers pass through, and I believe that in the fic there is a low-laying area of flat land on the south shore close to the head of the loch containing Hogsmeade, but aside from that it is possible to pin down the locations of every event thus far in the entire fic to a specific grid-reference. The story describes real, findable, places throughout, a policy I'm delighted to see Dunkelzahn continuing - if you were to come to this part of the world and meet up, I could lead you to the exact location I established for Hogwart, and if we were to head down south to Avebury I could show you the exact stone at which Harry's original transformation occurred.
In general, Loch Morar is one of the best places in Scotland that I can think of to hide a large magic castle and related small town, particularly in the section of land between Glenfinnan and Mallaig - the location even comes with a set of RL ruins, and for the 'lake' hey presto, you've got this whacking great freshwater loch complete with its very own loch monster folklore. It is also conveniently in the middle of absolutely fucking nowhere while being only a few miles from an extant real-life railway line, which conveniently features that exceedingly distinctive bridge. It's also conveniently placed with two thousand foot tall lumps of solid rock between it and every broadcast antenna in the world, certainly as of the mid-90s; Hermione's canon description of a radio set 'producing nothing but static' at Hogwarts is precisely what I'd expect a fully-operational commercial radio set to do there to this day and I don't believe anyone has ever got reception on a cellular phone out there. To get a radio (or mobile phone) signal in or out, you have to run it via a satellite; to get a radar sweep of the glen the loch lies in, you have to fly an aircraft fitted with that radar along the loch."
Long story short, anything below 2,000 feet above Loch Morar is behind the horizon for any military or civil aviation radar ground installation in Britain.