I've looked but have never found a frequency for a specific security company, they all seem to contract with a communications system. Try monitoring these 3 LTR systems, you may come across what you are looking for.
There seemed to be quite a bit of security type stuff later in the evening on the FleetNet / Silke system, but who knows what companies they are. As fast as those security companies come and go out of business, it could be just about anyone I suppose. I'm sure I've heard more than just first response on the Day Wireless system, that database was last updated in 2004.
I don't think there are any security companies on the 900 Motorola system. It is getting close to empty. I have heard rumours that Nextel bought it and will be shutting it down soon, so that they can use the 900 freqs for IDEN during the 800 rebanding process. The only real use on the system is from one or two garbage companies, Pizza Caboose, Thrifty Car Rental, Titan Freight, Romar Trucking and a few other businesses. I havent listened to it much in a few months so some of them might be gone as well?