The way an automotive shop would locate the leak is by removing the radiator cap and attaching an adapter that screws on in place of the radiator cap. Something that looks like a bicycle pump with a pressure gauge attached to it is used to "pump up" pressure to simulate pressure generated when the cooling system is at operating temperature, the leak is then found sometimes it is obviously leaking from a coolant hose or cracked housing, failed gasket or water pump, sometimes the engines cooling system needs to be inspected from below or inspected using a mirror and flash light. If the coolant loss is still evasive dye can be added to the cooling system that may indicate the source of the leak. A mysterious coolant loss may even be a sign of a head gasket that is not sealing well.
YES! It is called "loose bolt/head gasket syndrome. The carbon builds up inside and acts like a hammer while you drive hammering those head bolts right outta their jackets and boy is that expensive, people usually upgrade to a newer Northstar. You DEFINITELY do not want an engine from 1997 thru 1999, you ever wonder why you don't see more of those on the road? Loose bolt/headgasket syndrome got them BADLY!
Another bunch to watch out for are 2000 thru 2004 Northstars, YIKES those were junk, I mean they really turned into junk quickly. That God awful Dexcool ate the gaskets from within where you cannot see, the coolant mixes with the oil and like magic...U R on "Feetwood." I would bet on an engine from 2007 to 2011, GM seemed to have learned how to build them after they bored us to death with the same style for an entire decade! I drive a 2002 DeVille just turned 125,000, never started having issues until she got older, even then, the ugliest to date was the $525 heater blower motor and the $800 plus for wheel bearings up front but I am sure that was from where someone ran into me. I always take good care of my Cadillacs. You could even have a cracked block just tiny enough where you can't see it. Aluminium expands and contracts. AND, if you drive a Northstar like the church lady it will surely turn into junk by the carbon deposits building up inside the engine.
That high horse power Northstar was made to be driven, like a sports car need to be taken out and "blown-out" to remove the carbon deposits. If you drive one like I said chruch-lady style, you will be sorry. Use premium gasoline too guys, regular sludge is what I call that grade expedites your Caddy to the scrap yard. Remember: 1997 - 1999 Cadillac = checking account with lint and static left after ALL the repairs. 2000 thru 2004 are truly expensive pieces of junk, why a body control module is between $3500 and $5000 depending on how greed driven the garage is where you take them.The 2000 thru 2004 run thru sensors like crazy, the main ones, the crankshaft position sensors which cause the car to idle rough, intermittent starting issues, and stalling out in traffic.
The rear struts are expensive to replace also, I had a 1999 Cadillac Sedan deJunk that I bought brand new and the dealer had it more than I did! Those girls had bad rear knuckle assemblies that were $525 for each side which did NOT include labor, the rear struts were $750 each! That '99 was so bad I wanted to torch it and collect the insurance money, it was a crude piece of beautiful junk. I parked it in my garage when I bought the 2002 and just sitting in the garage the '99 developed blue smoke, a clicking motor, and when the guy that bought it came to get it, the rear wheel had froze up on it, Left a nasty black mark down my garage floor that took years to get rid of.
I tell everyone who likes a Cadillac: "Buy from a reputable dealer and make sure the car is sold to you certified. Girlfriend can and will take you to the cleaners." Ever wonder why so many beautiful Cadillacs end up on "buy here pay here" lots? The Cadillac dealer puts them on the auction block because 5 repairs from those girls would suck up all of their profit easily, Cadillacs are massively expensive to work on .... the cost is obscene!