Depends on where you live and what the crime rate is, as to whether your concerns are more real or an over reaction.
I know that when I originally moved out of my parents' home and into a unit in a leafy inner city suburb in Sydney, I was shocked at the higher crime rate and the effect it had on me ( car broken into stuff taken out of it, unwelcome folks coming to my door late at night etc.). Like has been mentioned, I changed my habits and left lights on, ran the TV on at times, installed a silent phone answering system - one that would answer the phone after a few rings, but not broadcast the message so folks outside could hear it work. Luckily I was only renting the place and things quietened down after a while - but I only stayed 4 years and bought my own place miles away from the rat race.
And even when I had moved into a lower socio-economically environment, the contents insurance went down instead of up. So much for living in the snobbier parts of town :dontknow:.....
I did security work for about 13 years. Your back to base sort of security system is only as good as the security company you sign up to.
If they don't pay a lot of attention to how their security folks respond to alarms, quite often they'll just drive past, flash a light, and see if the front door isn't smashed in. IF they have time, they may wander around the back and take a look.
All too many times, security companies will try and get by with as little overheads as they can have - and that means only a few guys driving around at night checking on alarms.
Their security people quite often get loaded up with a stack of checks to do each night AND respond to alarms, which creates a 'tick off the box' mentality on each shift.
In the middle of them going about the nightly checks they will get an alarm call, have to drop what they are doing and go attend that place in alarm. I have seen these guys drive up to a place, see the alarm light flashing, shine a light on the front windows and doors - see nothing broken and jump straight back into the car and call it in as checked, nothing adverse to report from the external. They then rush back to tick off the remaining checks they have to do for the night.
In many cases that is about all they can do, as property owners are often very reluctant to leave a set of keys to a security company, so they can enter the premises and really give a good check.
As for guitars, a good locked cabinet is something that may come in handy if you live in a higher than usual crime rate area. If you are using a spare bedroom as a studio, it may come complete with a built in wardrobe. This would be handy to install a lock on and keep the guitars in.
I don't know if there's a safer way to store your guitar aside from the obvious way of getting a tall and wide firearms cabinet bolted into a concrete floor, but one of those standing in your spare bedroom would be a bit of a giveaway, and I don't know if you'd want to store your guitars in a garage - where a firearms cabinet wouldn't look out of place - either.
In respect to people casing your place, a small external camera might help you establish if this is happening. You can get a small camera and attach it a hard disk recorder unit and run it on slo-mo for a full 12 hour surveillance.
But more importantly are you using an amp when playing guitar? If so, that is certainly advertising to all who walk past your place that you have a guitar and an amp - so maybe you might want to think of playing through a DI system into your computer and keeping the noise down, if that's a worry to you.
Passive infrared detectors can be too sensitive and installation is one thing, but getting them working properly is another! Too often they'll detect the moths flying around, or a mouse you didn't know you had, and forget about them if you have small dogs and cats that jump up on furniture and play around when you are not home.
Often a number of small changes and minor security measures will enable you to be more comfortable about the safety of your place.
A work colleague of mine, had a father who was a locksmith. Because he could use all the locks he wanted, their family home was set up with a series of locks - and good ones at that - which could all be locked when they left the house unattended. If anyone was to break into the place, they'd only get into one room, and have to leave the way they broke in. All the other rooms would be isolated from each other, by each door having a very good lock and door frame on them. The amount of time - and the noise they'd cause - trying to break into the other rooms would be unacceptable to the average house breaker. Obviously, if you were renting a place, that sort of security measure may not be available to you, as I'd doubt the landlord would consent to double dead latch locks being installed on all rooms.