I've had two bikes stolen in less than a year. The Harley my dad left to me and the Sprint ST I bought. The Sprint ST had a disc brake lock on it at the time of the theft.
I just did a deal on a Speed Triple and want to keep it, so I'm looking for an alarm, possibly a GPS tracker and/or immobilizer. I think the GPS tracker would be the best, it seems like most can set a 'fence' and once the bike is out of it, you're notified.
Your best bet is good insurance. No matter what you do, if someone is determined enough they are going to get your bike.
If they are outside, put them under old/beatup covers, if it is harder to ID the bike it is less likely to be taken. Disk locks, chains, alarms, GPS may stop an amateur thief and crackheads, but they are only a mild inconvenience to a pro. Locks and chains are easily cut with the right tools, alarms and GPSes are hard to hide and easy to defeat.
Try parking next to a more expensive bike, maybe they will take that instead.
I understand that insurance can't replace something sentimental like your father's bike, but if you are in a high risk area or unable park somewhere secure I don't see any better option.
If someone wants it, they're going to get it. Find a way to keep it indoors, "hide" it per Froggy's advice above, and get good coverage.
Also, consider security cameras on the parking area. Game cameras (motion sensitive) can work very well, and just about any of the cams these days download to either a cloud or a dedicated drive at the home. While it may not let you keep THAT particular motorcycle, it could definitely play a role in bringing the thief to justice.
Mine is the 2nd on the list, the "wired tracker", and I have the plug-in (OBD2) units on all my family's vehicles.
It's tucked under the pillion, next to my Power Commander. I have it wired to be constantly on, so it pings every 2 minutes, 24 hours a day. Yes, it can run the battery down after about 8-10 days. I use a battery tender when I'm not riding the bike. If you ride at least once a week, it's not needed, as it is a very low draw.
The accuracy is really very good, you can track from any web browser, and there are iPhone and Android apps.
I use it when I'm meeting a family member somewhere, I just open the app on the tablet I have mounted in my car and when I click on their icon I have the option to navigate directly to the target.
Geofencing tells my wife when I arrive to and leave from work, likewise it tells me when our daughter gets to/from work. It also displays metrics such as driving safety, speeding, harsh braking, etc.
I would agree with most of the above. If someone really wants it, the professional thieves usually know a way to get it (watched too many youtube videos on bike thefts).
For home, this seems to be the ultimate solution: http://www.artagosecureusa.com/#!bunker-security-/c1vg7 A friend of mine said he was getting one for his condo after his gsxr was stolen but not sure if he's gotten one yet or not.
This is not for a motorcycle, but it pairs with your phone and that enables/disables the security feature. I suspect this will be the next generation of motorcycle security also. https://www.smarthalo.bike/
Robertl, thanks for turning me on to http://www.artagosecureusa.com/#!integration-kits-for-motorcycles-/ce5d. They make a integration kit for the new Triumph!