I too live in a high-theft city. Our local Facebook page for stolen bikes is more than depressing (but helps recover quite a few!). From what I've read, bike theft is either professional and highly organized (truck or van, full crew targeting professional quality bikes for resale) or opportunistic (skilled individual with a few tools to break or cut locks, typically rides away on the bike, sells on Craigslist or Offerup for <$200). The vast majority of thefts are the latter.
Things I've learned:
* If at all possible, take your bike inside (at home too for me).
* Make it way easier for the bike next to yours to become a target -- so, parking in a group of bikes, with your bike locked with at least 2 U-locks through the frame/wheel/solid object. I have one regular U-lock, one mini (harder to get leverage to pop it off), and also a cable - all three go on my bike even if I'll be out of sight for only five minutes.
I also routinely park next to high-target brands - here that is Specialized and Trek road bikes.
* Record your serial number and have proof of ownership- pictures of you with the bike at the least, receipt of purchase preferably. In my city bikes with serial numbers recorded are recovered MUCH more frequently. I can even report the theft online which puts the number into a database for pawn shop owners, etc.
* Use commuter tags if your city has them - police here have also been able to identify a bike's owner by their commuter tag (we have ZAP
www.derozap.com)
I was reluctant at first to buy an additional U-lock but the way I lock up now makes it very difficult to strip any parts and hopefully not worth the effort to get the bike.
I'm sorry this happened to you and your partner. I would rather have my car disappear than my bike.