Is there any conduit/wiring running from house to shop?

There's an electrical conduit with 100 amp service but (we believe) we can't pull any other wires through it by code.

There are systems you can install for locations you describe and, you can tie them into your existing system if you like. Perhaps a temperature alarm would give you the added security you are looking for. You can set it for a high threshold to monitor for excessive heat. You could also set the lower threshold for low temperatures to warn you if liquids you keep in the shop may freeze....just another option

Thanks maryland. Do you have a name of a company I can check out?

Temperature Guard is a good mid range product. If you do a Google for "Temperature Alarm" you'll see a whole truckload of systems to choose from. They are easy to install and set up.

You need what's called a "rate of rise" detector which does more than just detect a high temperature which can sometimes be falsely triggered, but one that detects a sudden increase.

It's also true you need one that's suitable for the temperatures (cold and heat) likely to be encountered in an outbuilding.

They may make such a detector that's sort of standalone, or you may need to tie it into a cheap security system. If you already have an alarm (unless it's a basic big chain one that's not upgradable) you can tie it into that.

After checking around, it looks like all of these alarms require a land-line for a phone. Is that correct?

It is if you want them to call a monitoring service - if you're referring to an alarm system like an Ademco or DSC, that can integrate fire/water/low temp/security, the NORM is to have a phone line so it can contact a monitoring service (or you can set them up in 'pager mode' so they contact your cell, you respond with a tone, then it tells you in beeps what zone it is (typically, zone 1 would be fire)

You can also get a cellular product such as Digicell which connects to the panel and uses a cellular network to contact you but there are of course monthly fees.

Unless I'm misreading what you're saying. You'd need a run from the outbuilding back to the alarm panel or whatever, if you chose to have phone monitoring, then it'd connect to a phone line, if not, then it would be set up as a stand-alone system. If there was no phone monitoring service then I'd recommend (I would anyway) an external siren and strobe so passers-by could see what's going on.